You'll always get a certain percentage of bad items in any production run. We'd need a lot more statistics to see how many people have hardware problems compared to how many have none. Remember it's usually only people with problems who post on forums So I don't think there's anything inherently problematic with the N900.

Having said that, my first replacement from Nokia had a dodgy touchscreen which stopped working completely after only a day (looked like it was a refurbished phone and presumably it hadn't been fixed it properly). My second replacement looks to have been brand new and, so far at least, is working fine.
So in my experience Nokia's after-sales care is somewhat sub-optimal, but likely still better than a lot of other manufacturers.