Happy N900 owner
I've had a N900 since Tuesday and I'm loving it. Yes it's a bit of a brick because of the slide-out keyboard but no worse than the N97, in a handset that is more versatile than the good old Communicator phones.
In terms of hardware, it feels very responsive thanks to the Arm processor and the GPU. The touch screen is very nice to use, a lot better than the N97 and on a par with the HTC Tattoo. The stylus is useful when browsing web sites set in small size fonts as it's easier to hit the links with it than with your thumb, but the rest of the interface doesn't require the stylus. In fact, the only hardware snafu is that the headphone socket is located such that the headphone jack interferes with typing on the keyboard when you have it plugged in.
As a phone, it's the best one I have owned for a very long time. The sound quality is excellent and crystal clear. Signal reception is also very good. The past 3 handsets I've had would be unable to get a signal if I strayed more than a few metres away from the windows in my flat. The N900 shows maximum reception throughout the building. I suspect this is because it's very good at switching back to stronger 2G or 2.5G signals when it is out of range of a 3G signal.
In terms of connectivity, Wi-Fi also works flawlessly and the phone will automatically switch to Wi-Fi for internet services as soon as you are in range of a known network. In my case, as soon as I am back home, it reconnects to the home Wi-Fi and disconnects 3G services. This can have an interesting side effect as the N900 can also work as a Skype phone: configure your Skype account and it will give you the option to call Skype contacts through VOIP rather than a standard call. I haven't tried that yet though so I don't know how well it works.
Push email is very easy to set up and it knows the defaults for a number of well known providers, including Google, in which case you will get a simplified wizard. Calendars can only be local at the moment but the interface suggests that shared calendars are on the roadmap and should be available at some point.
When it comes to software, Maemo is indeed based on Debian and you can add more repositories to get access to more software. In particular, you can enable the Extras repositories to have access to admin tools such as an ssh client and server. What this also means is that, contrary to a lot of network branded handsets out there, whenever an update is available in the Maemo repositories, it will offer you to update your system. So any bug fixes or new features will make their way down to your handset.
If you want to develop your own apps, there is a whole development kit that you can install on Debian, Ubuntu or Fedora and that enables you to write Python or C++ apps (sorry, no Java). Or you can simply write good old Python (2.5), Perl (5.8.3) or shell scripts. You can even do that directly on the handset: it comes with a terminal app, it's got vi installed and if you really miss syntax highlighting you can get vim or PyGTKEditor from the Extras repositories.
The N900 is not for everybody but it's a geek's dream, in addition to being an excellent phone.