I bought my N810 in September 2008, and got my N900 a few days before launch. The pros and cons of either device seem to me to be mostly a matter of trade-offs, although the N900's hardware is certainly superior in many ways.
My thoughts:
- The N900's overall build quality seems better, although it's all plastic (I like brushed metal )
- The web browsing experience on the N900 is vastly improved, in terms of speed, interaction, and (for the most part) features. It's still much less convenient than a netbook or laptop, but I had to spend a few weeks using the N900 as my primary computer when my desktop was broken, and it did the job admirably in that regard.
- The N900's keyboard is smaller, and less terminal-friendly, but (IMHO) the tactile feel of the keystrokes is much nicer, and the lack of D-Pad means that it is more centered than the N810's. A cross between them would be excellent, but I personally prefer the N900's.
- The N810's kickstand is awesome, and the N900's is almost useless. I especially miss being able to stick my hand through the kickstand (flat) for reading/watching one-handed without dropping it. If the wimpy kickstand bothers you, keep an eye on Fake's laser-cut stands (search for the thread), which are quite nice, if not a full substitute.
- The N810 had USB host, and as you probably are aware, the N900 does not. This likely isn't a big deal for most people, but I did love being able to plug in a flash key. (Of course, now if I need to when I'm out, I just plug it into the N810 and send it via Bluetooth to the N900)
- The screen quality is fairly similar, although the N900's is certainly brighter, and maybe a bit more vibrant.
- The physical screen size is a trade-off... I often use my N810 to read e-books, so I can read whenever I have a few minutes without packing a thick novel around; the N900 is too small to comfortably do that, although it's still large enough to be quite usable. Of course, the nice WVGA resolution is less useful in some ways due to the relative sizes of screen and fingers, but Nokia has done a good job of making it work in terms of interaction. If you have less than perfect vision, you may need to zoom the browser (which is easy to do, in three different ways), but everything else should be readable. Despite the more smartphone-y size, however, it is still very much a two-handed device.
- I can't really speak to gaming ability, because I don't do much of that. The arrow keys are certainly more than sufficient for the tasks I do, and are physically much easier to manipulate than the N810's D-Pad, but I don't know if it would be a suitable substitute for your use-case.
- The Notes app has seen a few improvements (such as being able to save in plain-text), and a few features gutted. Overall, it's kind of "meh", and you're probably better installing LeafPad or something else, for most uses.

Overall, I've been really pleased with the N900, and it makes my N810 jealous, but it's hard to know whether recommend it to someone I've never met. It's seen a lot of improvements, but it's also still rough in some areas. You should probably see if you can find one to play around with for a few minutes, if you can. I think there was a thread somewhere on here to post the locations of forum users - you could check that and see if there is someone in your area who could give you a hands-on demo.
Cheers,