I would say, presuming you don't install anything that will break its phone functionality (I doubt you will, unless you start getting extras-devel stuff), the N900 is perfectly fine. fMMS works fine once set up for MMS support, so calls, texting, picture messaging and internet will be just fine. I honestly got my N900 as a portable laptop for connectivity to the internet on the go, so I barely use it as a phone. But it makes a great netbook. That's not to say it's bad as a phone. The only currently existing gap is the lack of a full portrait-mode virtual keyboard for everything. There are a couple of text-messaging-in-portrait-mode apps for just texting. If you want browsing completely in portrait mode (IE absolutely wouldn't be able to live with entering urls and passwords and stuff with one hand) you may want to hold off a little. HOWEVER, people are currently working on a universal portrait keyboard. It was made by Nokia originally I think, then not included for some reason, and now it's in semi-usable form in extras-devel. It will probably be working eventually, but it may take a couple months. In the meantime, I find opera mobile is almost as good if you want not-always-full pages and no flash support in exchange for having a built in vertical virtual keyboard.

As for portrait mode itself, with the latest firmware, N900 does portrait mode on almost everything. And while some people complain about it being half assed and broken or whatever, I think it's fine for most things. It is broken for stuff like media player, but not for most things, and you can generally use the media player without turning on portrait mode even when you hold the N900 vertically. You still have to Cttrl+Shift+R to get it everytime you open the apps, as I think you know (except browser, where you can enable it automatically), but that's not hard or annoying in my experience. Really, unless you're completely obsessed about portrait mode, and can't live without it, I think the N900 won't cause problems, and you said you thought you could get used to it, so that's not likely to be a problem for you.

I think it's worth it, honestly. In the entire time I had this device, I was lightly annoyed by one aesthetic change in PR1.2, had a couple problems solved by rebooting (one involved accelerometer, and I am not sure if reboot and battery removal fixed it, or something else), and only one problem currently being discussed in another thread with GMail loading as mobile all of a sudden. If you want it as just a phone with full internet, go with it. It won't disappoint. If there's anything else you really need, that may or may not be possible, but it probably is.

As for navigating the UI, it's perfectly fine in my experience. I can barely do stuff in X-Term, and simply stared at it when I first launched it because I had no clue what to do, but everything was very intuitive outside that. You don't need to think to use the majority of the phone. Almost nothing you need to use requires knowing much or being tech savvy. Bringing the device to full potential does, general use doesn't. When you find some specific thing you want to do you'll probably be more than happy to learn that one thing. That's mostly how things have been going on my end. Using most programs, as well as getting them, is perfectly doable too. Unless the program requires some level of savvyness, from OpenSSH for basic savvyness to stuff out of my league, it shouldn't be too hard to find your way around. The app manager is perfectly intuitive too. The only non intuitive thing is adding/enabling repositories, but there's step-by-step explanation on this site's wiki.

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Price-wise, I got mine barely used second hand for 450$ including shipping, not the best deal but not the worst. Dell sells new ones for 470-ish, sometimes they have sales for 430-ish. Either wait for one of those deals, if you want this device now, or save up for its successor. Unless you're fine with 470, or getting it second-hand.