i got this as my first smartphone and it has been awesome, I debated between this and the iPhone 4 - the Nexus S won out with a bigger screen (I find 3.5" a bit cramped), and Android just seemed more exciting. Design wise, I like the shape and the feel in hand, and the curved screen I have found to be somewhat helpful in outdoor use (deflects the light different to a flat screen). My biggest issue is the plastic - its not really poor quality but some metal or Nokia style material would be nice.
As for software, Gingerbread was good, though not as polished as iOS - but I have updated to ICS and it is fantastic. The new OS is smooth (I had concerns the processor wouldnt be up to it), with tweaks and touch ups to the stuff that did work, and overhaul of what didnt - i'm thoroughly impressed. Battery life also doesnt seem to have suffered, I still get a solid 48hrs of fairly constant use (I, don't game much however).
The only thing that dissapoints is the camera, bit slow and not particularly good quality.
The Nexus S is a great phone, and i'll be sticking with Android as long as Google keeps putting out updates to the Nexus line.