I bought a Nokia N810 which is the predecessor to the N900, also based on Linux, but without the phone capability - essentially a very tiny laptop. I have not needed to understand Linux to use it although I get the impression that I could make it work more to my liking if I did understand Linux. I suspect that's what you'll find with the N900.

If you like the way it works out of the box and you can tolerate the limitations in the number of apps (and the lack of polish in many that are available), you should do fine. OTOH, as was pointed out, Android, right now, has a better selection of apps and more phone features.

As far as using the phone for browsing, you need to understand that the browser is limited versus the browsers that are available for laptops. For the most part you won't notice this but some sites will not work or will not work well. The biggest problem you will find is the size of the screen. My N810, which has a larger screen than the N900, is fine for looking up something but tiresome for extended web browsing. Also navigation within a website is often difficult because of the size of the hot spots.