For what it's worth, with my previous smartphones (Nokia E61, E90 and iPhone 3GS), I only used a few dozens of MB per month. That was mostly spent on email, Twitter and a bit of web browsing.

With the N900, I've only had it for 2 weeks but am already at 635MB of downloaded data (and 35MB sent) - that includes both 3G and wifi. Part of this is probably the new toy excitement syndrome. But in general, the N900 is just a fantastic Internet device that pushes you to be online all the time.

IM is always there and so seamless that you just use it without really thinking about it. Web browsing is a real joy. YouTube videos display inline and just work - I've found myself watching much more embedded YouTube videos than I did with my previous phones. The FM transmitter is great too - I've been regularly listening to French radio streamed over 3G while driving thanks to it.