Well, after two weeks of SGSing in place of my trusty N900, I have a few things to note. These are my observations. Some are factual and some are subjective. This is supplied for those who want to hear about the comparison and not for those who want to troll about the subject.
FORM
The first thing I noticed about the device is its form. It is thinner and sleeker than the N900 but that is down to the absent hardware keyboard. It feels less sturdy than the N900 but doesn't seem to be the scratch/dint magnet that is the iPhone4 (yes, test driven that too).

BOOT
There is something quite satisfying about all new hardware. Turn it on and it is pretty and shiny and new - and fast. The SGS boots promptly but so did the N900. Not put them head to head but I am writing about the feel of it all and there is no obvious winner here.

SETUP
The initial setup of any Android phone is a little irritating. Because it is "Google-powered", so to speak, it wants you to link the phone to your account. Nothing to shock an Apple user but seems like an intrusion coming from the N900's "do what you want" background. Mind you, the PR1.2 text sign-up thing kinda knocked that for Nokia.

JUST HOW YOU WANT IT
Well, neither device comes out of its box knowing who you are and what you want from your device. I suppose that might start to happen when OSs start syncing your laptops, desktops, tablets and mobiles a la Chrome/Chromium but I, for one, am not sure I welcome that.
In any event, part of the fun with both devices is finding out what it can do and how you want it to do it.
The Samsung "bloatware" is much whinged about but, in truth, is not that bad. Some of it is even helpful. Again, N900 users might not like the fact that you have to root your device to remove the Samsung content but it is not intrusive in the manner of Vodafone UK's Live! buttons all over the place.
Android is flexible. Far more flexible than many might realise. It occupies ground between the walled garden of Apple and the anarchy (cough) of Maemo. You can put the device in debugging mode so anything can be shunted between the device and computer and you can install apps that are not signed if you choose to. No nagging reminders - you set your setting and do as you will.
For the everyday user, the SGS feels free and easy. It is only when you decide that you want to see if you can <insert insane N900-only type project here> that you might have a problem.
I am neither a developer (though I live with one) nor a "I want the one with the wifis and bigger GBs. Can you set up my bluetooth for me?" sort of user. I am a geek and a tinkerer but not one who wishes to run my device from a command line alone.

SCREEN
The Super AMOLED is impressive. Really impressive. It isn't flawless and you can see, wait for it, pixels. Sorry iPhone 4 users. Vile isn't it? But it is impressive. Sorry N900 users. Your screen is nice. But it just doesn't hold a candle for clarity or ease of use. To test this, I tried to read news websites (guardian and bbc) for an hour on each. I left a day between reads. The N900 caused no physical pain but I was sure conscious of staring at that screen when I was done. The SGS is totally painless. It's true. And video looks stunning.

INTERNET
The SGS has the same range of internet options as the N900. It does support wireless n but poorly for now. I understand that froyo (Android 2.2) will improve this when it is dropped on 23 September. We watch and wait.
The performance through the SGS's standard browser is slick and fast. It is definately faster in almost all types of rendering than the N900. This was tested over wifi and HSDPA but, where the SGS really shines is when getting data over GPRS and EDGE. The N900 will get there in the end but a test of page-for-page rendering just using 2G/2.5G sees the SGS win hands down.

SOFTWARE (PC syncing etc)
Oh dear. Well, the N900 and SGS have something in common here. Shockingly poor syncing support with their respective bits of software. OVI Suite/PC Suite and Kies (for the SGS) are both slow, bulky, unreliable and, in just about every sense, tedious and unfit for purpose. Samsung has promised to fix this but I feel wary of phone manufacturers making pledges. Can't think why I got so cynical.
Much like with the N900, use wireless syncing methods and USB Mass Storage modes for each and you can get rid of those nonsensical programs for good. Apple needn't look smug either after Ping and the most recent bloat-tastic iTunes roll-out. Seriously folks, start making an effort, eh?

SOFTWARE (Apps)
"I don't want them." "I don't need them." "My phone is nothing without them!"
Whatever.
The reality is that we like to have distractions, tools and geekery on our phones. The range is from VPN, P2P, live streaming video, chat and the like through to "push button and get fart noise". Take your pick. N900 users have been saved from boring pub times by the very hard work of this community's team of geeks-in-residence. God bless them too. Without them, the N900 is a very expensive piece of toast that will get the street-cred of a pony tail and mint condition Star Trek phaser. [I admit I did have the LCARS theme on my N900 though].
There is stuff to play with and to use on the N900 and it depends on who you are and what you want to use your device for. I ran a hockey playoffs live show from a town with no bleeding wifi and nowhere to plug in on my N900. I get my work documents where and when I need them. I go geocaching and sometimes play mobile games when I get bored.
I did all of that with my N900. Thanks to some good people.
I can do it with the Android. For the same price (free). A little quicker. A little prettier.
The winner here is the SGS for the pure fact that, as a user rather than a dev, I go to the market place. I get want I want or need. It works more often than not. If I buy it but bin it within 24 hours, I get my money back automatically (yeah, take that Apple!). I can see what arch-androiders like Gina Tripani and Leo Laporte are using. Or just my mates, if they share. I can sort my apps in a more social context. And the devs are getting paid reasonably for their work. Its win win, baby. Sorry N900. OVI Store and Nokia have failed you bad.

KEYBOARD
I suppose this warrants a special consideration given the two factions that exist on here. If you want a hardware keyboard (in the UK), you will have to wait for the SG Pro (November, I understand). Nevertheless, Swype is worthy of a mention. Sure it is odd when you first give it a go but it's so very easy to pick up and it is extremely quick and intuitive. Unless you have tried it, it is hard to explain how it compares but for quick typing on the fly, it is really quite remarkable. Test drive it in a shop and see what you think.

FOR A DEVELOPER
Ok. Not one myself but my partner is. The big turn off for the N900 SDK seems to be that you have to dig around for all the bits you need and, despite the fine efforts of this community to pool knowledge and advice, Nokia has not really pulled out all of the stops to make Maemo quick and user-friendly.
The attraction, it seems, of the iPhone's SDK is that it is a one stop shop for everything. It's all there. It's workable entirely offline and it is well supported and has a clear (now!) set of parameters as to how you can do things. It may be restrictive and walled but it is user-friendly.
The SGS SDK is also a one stop shop. The support is being offered by Samsung and by Google. Admittedly, Alex is just starting to use this (after putting down the beloved iPhone 4 to play with the SGS) and is little beyond Hello World for now but the whole experience seems easier. Time will tell and, I have no doubt, this community's devs will try and test it themselves.
What it means for end users is that there are more people building more apps and monetizing more readily on the Android platform than for Maemo or MeeGo. This is good news if you want your phone to entertain you for longer.

THE FUTURE
Android is on the up. How on the up is a matter of dispute in the tech world with Apple and Google slinging figures about without any real meaning to end users. Nevertheless, Samsung are being fairly straightforward about the update roadmap and the Galaxy brand (a bit like the Nokia N Series) is not stopping here. The Tab is on the way and a Pro edition of the phone is ready for release soon.
Froyo (Android 2.2) is dropping on September 23rd. Gingerbread (Android 3.0) "may" get an SGS rollout in the fullness of time. Samsung have said "may". Who knows? But, like the Maemo community has done, the Android community is already thinking about how to force that into being.

VERDICT
The SGS wins for me. That's "for me", ok? It is a delight to use and has entirely reasonable battery life. Samsung are dedicated to it (if not to the ropey Kies program) and it shares the swiss army knife nature of the N900.
The N900 is not dead to me. I will keep it and use it for various things. It has one feature that I sorely miss. The FM transmitter on the N900 allowed me to play podcasts via the bathroom and kitchen radios. I miss that like crazy but I can work around it. The SGS 3.5mm jack is annoying as it won't allow calls to be routed through it unless there is a mic on the headset. This needs tweaking for me.

If you are trying to decide, I hope my experience will inform or assist you. If not, thanks for taking the time to read it, anyway.

Please respect my right to publish my views and don't get jump and defensive about our Nokia black sheep device. I love the N900 but, for now, I love the SGS so much more.