I bought the N97 (stop laughing at the back), and I must confess to looking at the N900 a few months later when it came out with lust, but I was trapped in the N97 contract, so the N900 wasn't an option on contract, but I thought of buying myself one.
Luckily (in hindsight) the pitiful support provided by Nokia for the N97 convinced me that there was no way Nokia deserved any more of my money, so I stuck out with the N97 for over a year before reaching the end of my tether and buying a Desire Z (I'm still in the N97 contract).
I can't compare it to the N900, but I can compare it to the N97. They keyboard is okay, but I preferred the N97's, so I expect the N900's keyboard was also superior. The camera is also not up to the 5mp camera in the N97. It's okay, but nothing special. Then again the N95/N97's Carl Zeiss camera was rather exceptional, so not quite measuring up to that isn't too much of a surprise, and the Desire doesn't have a lens cover that scratches the very thing it is supposed to protect!
Software wise, well I don't need to tell you how much there is available for Android. Lots! Custom firmware, yup, plenty of that. Forums and grass root hackers, yup, plenty. It's as much yours as you want it to be.
Having said all that, I haven't even rooted mine. It works fine. Way better than the N97 which invariably required a wipe and reinstall every month!
Development environment, simple. Unlike Symbian the instructions and documentation are clear, the package links are all together on one page, they all download, and they all work first time on Windows, Linux and OSX. Plus completely free. I tried a few times with Symbian and Qt, but the Nokia website was so disjointed with instructions a version behind and multiple packages on multiple pages, I never got either to work! You know you should give up when even the simplest built in example apps throw up cryptic missing package messages.
Anything I miss? Well oddly enough (given how people slate resistive touch screen) I miss the accuracy of being able to use a fingernail to tap a link. Now I have to zoom in and use the side of my finger instead when there are many links close together (or use the little trackpad joystick thing). Only other thing I can think of is the headphone remote only has pause, forward and backwards. The Nokia one also had volume up and down.