Thanks very much raphb! Some of the negs:
The N900 excels at task switching, the communication apps are good enough, and music playback and photo browsing are better than on any previous Nokia.
Text input is still hugely neglected by everyone, Nokia included.
The marketing people at Nokia are obsessed with photo sharing on the web, perhaps because the expensive marketing consultants they hire tell them that. Or focus groups are loaded to "rich media". But the "content" that most of users "generate" is text.
Nokia is very coy about battery life, but says the N900 should fulfill the goal of getting through a day. Talktime, I was told, should be comparable to other high end devices. I'll be astonished if that's true. Nokia's own E71 sets such a high standard here for the entire mobile industry, I can't imagine how an OpenGL device pushing five times as many pixels around is going to get close.
The browser is based on Mozilla. It's much improved on earlier versions, and the performance here brings it into the Android and iPhone class. But somewhere near the back of the class, where the slow children sit. It doesn't feel quite as slick at loading pages or scrolling as these rivals - while you really need a heavy site to slow down the 3GS, but my dusty personal archive (http://andreworlowski.com) took about 20 seconds to load on the N900, which felt like ages.
It doesn't support Java, but it does support full Flash, so YouTube works fine. It's stolen some of the iPhone's gestures (double tap to zoom) and a really wacky one where you corkscrew clockwise to (slowly) zoom, or anti-clockwise to zoom out.
The always-on data capability ensures that. A few may opt for the N900 as a primary phone - perhaps people who don't make too many phone calls.