I am enjoying my Nexus S for the peace of mind it brings: knowing I'm running stock Android and that I'm going to get future updates first.
Having said that, I do wish the experience wasn't marred by issues and glitches (specifically with the random ringtones, and the occasional, yet nevertheless frustrating, screen scrolling and icon issues). I'm looking forward to the next bug fix update. Fixing these issues I predict will go a long way in making me really happy.
I'd like to add: One aspect of the phone, and of Android in general, that I feel sorely about is the keyboard. No matter how one twists it - for example, one might say it's improved over previous Android keyboards, which is true - it pales in comparison to the accuracy and response of Apple's keyboard. This is not to start some immature Apple vs. Google flame war; anyone who has typed extensively on both platforms will understand the difference. It's also a shame Google seems insistent on copying even Apple's mistakes, for example, the lack of customization and custom auto-correct features. I can't wait until Blindtype is implemented. It's sorely needed.