Phone app:
Strengths: Phone app integrates skype, sip, google voice, and phone all into one app. Conversation app features similar integration with SMS and IM.
Weaknesses: Always seems to auto-rotate at exactly the wrong time, right when i'm trying to click answer. Poor logging.

Mail app:
Strengths: None
Weaknesses: Slow as a dog in some configurations.

Web Browser:
Strengths: Fast. Smooth. Accurate rendering
Weaknesses: None really, I feel Opera Mobile gives better readability, but at the cost of accuracy.

Media Player:
Strengths: Wide variety of supported formats. FM transmitter, and TV-out are quite useful.
Weaknesses: No gapless playback, Media Player widget often doesn't respond to play. No equalizer.

Hardware: Front camera is garbage -- unusable even in a reasonably well-lit indoor setting on all the units i've seen.
Shiny ring around back camera ruins flash photos.
Battery life is not so great, but spare batteries are cheap.
Mediocre wi-fi range -- better than my laptop, not as good as my roommate's laptop.
Resistive touchscreen -- I like the precision, and the ability to take handwritten notes with the stylus, but many of the WebOS and Android games are mostly unplayable due to the lack of multitouch.
Kickstand is very nice, but device is poorly balanced with keyboard extended. I mostly use it by slipping my finger through it to prevent dropping.
Magnetic sensor for back cover stopped working on my first unit, making microsd unusable.
Stereo speakers sound nice, considering their size.
Hardware keyboard feels nice, and is easy to use.
No compass.
Plenty of storage.
IR out -- poor range, no IR in, so you can't just use it as a learning remote. I've never used a television that was actually listed in the irreco database. Xbox remote and apple remote work.
Only 256 MB of RAM. Default swappiness set too high.


Other software: This is where the N900 really shines. 3rd party apps make up for many of the weaknesses in stock software, such as Symfonie music player's equalizer and Faster Application Manager is a much faster application manager.

Many of your favorite linux apps have been ported, if not you can run them in a chroot, or if you're really brave you can add debian's lenny repositories right into your sources.list and run them natively just watch out that they don't conflict with your existing maemo software and be careful not to fill up your rootfs (rootfs space problems easily solved by copying it to /home using an easy script). If you're running off an alternate home partition, you can add the SDK repositories and write/compile programs directly on the device.

Powerful CLI tools.

USB hostmode available, convenient for external storage, mice, keyboards etc. Mouse and keyboard are convenient when hooked up to t.v. (I prefer to use bluetooth mouse and keyboard though).

MobileHotSpot features wireless tethering as well as wired. It can even tether your computer to wifi, which is convenient for me given the godawful range on my laptop.

Easily overclocked to nearly double its rated cpu speed if you need more power in a pinch.

Alternative operating systems can be installed to MicroSD or internal storage.

Awesome multi-tasking window manager. (Can even install IceWM straight from debian if you'd like to use a normal desktop Window manager with your mouse and keyboard)

Built in backup software not compatible with pc-based backup program. Built-in backup refuses to restore from internal storage, will only read from microSD.

Extreme scarcity of commercial software. Ovi store is a joke. But, the repos contain nearly everything you'll need. If you're used to linux, pretty much everything you'll want is readily available.

Ultimately, i'd say it's the most flexible and freedom-respecting phone on the market. Root access is readily available from the app manager. It's YOUR phone, not your carrier's, not the manufacturer's. I've used many other phones, and I like the N900 the most, by a lot.