This is the review of an experienced user. I'm not the guy who has used a device for 1 week and writes a review of it, I have been using this device for at least 7 months.
You can view the device specifications here.
Physical stuff
The phone is pretty heavy. It's way thicker than the iPhone and heavier too. But I don't think it would be way different than keeping any other 3.5 inch phone in your pocket. It feels good in your hand, no sharp corners and such. The sliding mechanism which gives you access to the qwerty keyboard feels sturdy. I've been using the phone for a long time, but the mechanism isn't loose yet. The phone has a full qwerty keyboard, which is useful, though I would've preferred a row completely for numbers and special marks. The lens cover is very good, you don't have to clean the lens, and you know it's protected if you drop the phone or something. The screen doesn't get scratched very easily (I've dropped it many times on a wide range of schtuff). The scratches are unnoticeable when the screen is on.
Screen
The phone doesn't have a capacitive touchscreen, which was a disappointment to many. But unless you're used to multitouch pinching to zoom in the iPhone, you won't probably miss it. The screen itself is very bright and sharp, 16,7 M colors. The touch sensitivity is really good compared to other resistive touchscreens (so I've heard). I don't have anything to complain about it except that the screen gets greasy at times, because Nokia didn't want to use material that rejects grease.
Camera
The camera is really good. It's practically the same camera that's used in all the other N-series phones. Sample pictures can be seen here. Video capture is decent, tho if you move the phone around quickly while you're recording you'll find problems in the video afterwards.
System
The N900 uses a different operating system than all the other nokia phones. The Maemo operating system is based on Linux, and is as free as Linux. You can customize it as you want, it has the same basic Linux folders, the command line commands are mostly the same etc. You can overclock it (from what I've heard from people who have overclocked, the phone runs more smoothly and mostly without problems), change your kernel, its settings, you can install other operating systems (I've seen someone run Android on it) and they work, because the chipset N900 has is very common (basically the same as in iPhone). The system is very flexible, you can play with it with root access. hmm... so you better know what the hell you're doing when you're doing it!
Another thing to talk about is multitasking. You can run a lot of apps at the same time, and they'll all stay active in the background (unlike in iPhone). Multitasking is one of the strengths of the phone, you won't notice it untill you switch to another phone and you realize you can't have too many programs active at the same time.
Phone capabilities
The phone capabilities have been said to be limited comparing to other smartphones, which is true. But you have to remember this is the first Maemo device with phone capabilities. Others have just been internet tablets, so you can say this is an internet tablet with phone capabilities. Here are call and Email/messaging features:
The SMS messaging are shown as threaded conversations, which is wonderful, just like IM. Skype is wonderfully integrated into the calling application and you can call over wifi and 3G. IM and SMS are both in the same application. The IM part doesn't have many features, but you can download Pidgin Internet Messenger from the repositories, it has the same features as its other Linux versions (and it's being updated frequently). Email client is decent, I have nothing to complain about.
Software
The N900 doesn't have a lot of software for it. There is the Ovi Store by Nokia, which is more of a joke than an app store, then there are the Maemo repositories. The repositories come in 3 levels, Maemo extras, extras-testing and extras-devel. There isn't a lot of software in them, but the ones that are in there are pretty nice. Applications are either installed through the applications manager or through x-terminal. The application manager acts similarly to synaptic package manager.
Web Browser
The N900 comes with a web browser called MicroB, which is basically firefox modified to fit the phone's capabilities. You can install some of the firefox add-ons like AdBlock plus on it. Actually it's very good that you can get rid of annoying ads while browsing the web. Another nice add-on is flashblock. While the N900 does support flash, but numerous flash objects on a webpage slow the browser down. Flash 9.4 is supported. I don't know if they'll some day provide an update. But there is an app that lets you fool websites by letting you manually tell them which flash version you have (you can fool Grooveshark to work, for example). Youtube videos run, but not smoothly. Most of the time they lag at the start but then catch on and run somewhat smoothly. Other browsers available are Firefox mobile (not mini), Opera mobile (not mini), Chromium and Iceweasel (debian firefox). The browsing itself isn't as smooth on sites like Facebook which is a shame. You can see a sample of the browsing smoothness here. WARNING: the guy who is using the browser in the video doesn't do the zoom gesture right or know how to swipe calmly, so that's why it looks retarded. The browser supports pretty much everything but Java (and silverlight, but who gives a crap about silverlight, really?).
Media
The media player is good, you can play music, video and internet radio. MPlayer is also available in the repositories, you can install the GUI "KMPlayer" for it (you can view video streams with it, I sometimes view TV channel streams). Various other format support packages are available in the repositories. Geotagging is available for photos, the photo viewing application is decent, you can sort and view by tags and by the date they were added.
Other interesting apps
As I said, there are fewer apps than on other platforms, but the ones that are available are really good. Some nice apps are Evopedia (offline wikipedia viewer), Easydebian (you can run a debian distribution on top of Maemo, you can use all the debian software like GIMP, OpenoOffice, etc), software to stream media from your computer, local web server software, VPN software, and lots of gaming device emulators etc. The N900 has an infrared port as well. There are apps which utilize it. You can use your phone as a TV remote, you can play emulator games using wiimote and you can issue camera flash commands through the IR port. The device comes with a PDF reader, you can also install various other office applications and document viewing applications like e-book readers and comic book archives. The availability of OpenOffice.Org leaves no need to buy documents-to-go to view MS office files, which is rather convenient. There are 2 applications to view Stardict dictionaries. The one I use is QStardict, and I can download free Stardict dictionaries to it. A torrent client is available (Transmission) too.
Programming
For a programmer, N900 has a wide range of capabilities. There is QT (including PyQt), PyMaemo and many other frameworks which are not officially supported but are available through the repositories. Java is not supported, sadly. But OpenJDK is available, and you can somehow force-install a trial version of Java on it, too. Icedtea 6 is available, Ruby on rails is also available. Shell is open... Possibilities are endless...
Smoothness and user-friendliness
The interface is quite smooth and consistent. You'll find the application options mostly in the same place, the dashboard, which allows you to switch between tasks or end them works well. The system runs smoothly unless you're running a heavy task like installing/uninstalling apps through the application manager. The core applications such as the phone and messaging applications are ready for use almost immediately after launching. The desktops are convenient as well. You can have up to 4 desktops and switch between them. You can fill them with widgets, all kinds of shortcuts and bookmarks. You can watch some videos on youtube to see how smoothly it works. Stuff like lagging youtube videos can easily be fixed by apps like Cutetube, which brings the whole youtube in one app with lots of functions.
Also, you can dual boot Maemo and Meego, the newest operating system of Nokia. So, even if Maemo dies off completely and nothing new comes for it, you can still thinker with Meego on it. I doubt Maemo will die off, though. The Maemo community is active and full of developers who want to play with their toys even though Nokia just pulled an atomic wedgie by not officially supporting Meego on the N900. The mere fact that the community has created a Meego for N900 stands for what I said earlier.
Any questions about the device are welcome.