I've had the Nexus S since launch and I have to say it's been the best mobile phone I've ever had, it's a great phone to use, the hardware is still sufficient to run the newest version of Android and you can now pick them up very cheap.
The phone is very slick and smooth to use especially on Android 4.1.1 (Jelly Bean). It's ultra stable, mine would frequently go for a month or more without a reboot and most of the time only was switched off due to it needing to be switched off for something rather than it 'needing' a reboot.
Needless to say it's also been excellent for updates always getting the latest version of Android quickly.
Battery life is very good and I've had mine go for two days on a charge, although I've just ordered a replacement battery for mine because this has suffered from over a year of heavy use. I was pleasantly surprised to find out that this was only £10 for a genuine Samsung item which is also very good if you need a replacement or just a spare.