I'm running a Froyo based AOSP ROM on my Samsung Droid Charge (in other words, a Nexus S in a Droid Charge body) and had the opportunity to do a "side-by-side" test with my phone and a Nexus S 4G. I cleaned off some app short cuts on the Nexus, switched to a regular wallpaper and rebooted both phones.
The Nexus S rebooted faster than mine but that's where its victories end.
My Charge was smoother when scrolling through home screens and while scrolling through the app drawer. The Charge also transitioned faster and smoother from the app drawer to the home screen. The Nexus was slightly stuttery.
Battery life is another issue. I have no idea how long a Nexus S 4G lasts. It probably lasts longer than mine but if I was stuck with one, I'd probably wish it would die faster rather than have to go through the frustration of dealing with the slow and fragile WiMax signal. 700mhz LTE is a very spoiling experience.
With moderate use, in an LTE fringe area, I get around 14 hours until I hear the 15% battery alarm.
All in all, the only real benefit I can see coming from Gingerbread is a supposed extension of battery life; the rest is just aesthetics really. But I do have a sneaking suspicion that if the phone wasn't originally designed for Gingerbread, you're probably not going to see much improvement with an OTA update. I could be wrong though.