For me the most direct comparison would be the Motorola Milestone / Droid.

The hardware is very similar and they both do a lot of the same things. Where the N900 is very close to Linux though, the Droid is a bit further away as it runs Android.

Both have keyboards, very close resolution, the same processor (Although the N900 is slightly faster I believe), similar amounts of RAM (Although the N900 has virtual memory too) and really, despite things like the FM radio/transmitter, it doesn't seem like there are huge differences.

The real thing it comes down to is software. The N900 runs Linux and that's great and dandy if you're familiar with it. You can do a hell of a lot.

This is the way I've come to really compare them -

If you want a mobile computer with phone functionality - Get the N900.

If you want a phone with mobile computer functionality - Get the Droid. You'll be able to do most of the things the N900 can, but you'll have to get to that with apps from the Android Market.

The Droid comes with Google Nav too, which can be a real feature if you drive a lot.