I have been running the leaked early beta release of Gingerbread (Android 2.3.2, I9000XXJVE) on the Samsung Galaxy S for about a month now and I can tell you that it looks really really good. The build is very buggy, so I would advise anyone who is tempted to do the same to hold on till Samsung puts out an official stable release (it will also not play nice with your warranty should you chose to run it on your phone).
There is little by means of cosmetic changes that is noticeable to those who are not detail-oriented and it is largely consistent from the 2.2.1 days. The notifications pane gets a new coat of paint and new icons. The apps stack is a straight carryover from 2.2.x, though I guess they may change it by the time a stable release is out. Almost everything worked out of the box for me and installation was done using Odin, which is never a comfortable thing.
Before I flashed to this build, I was upgrading to the CM7 beta release for the SGS, but after I set up the phone it figured that it just would not get on the mobile network. CM7 was looking really good till that point. I will probably move to that once they put out a stable release.
The Gingerbread release by Samsung, though, fixes most of the bad issues of previous releases of Android on the Galaxy S. The boot process is really really fast, unlike previous releases where you had to wait forever to get the phone booted into an usable state. The fact that I have not rooted it or applied any lag-fixes on it should tell you how good it is. Neither Eclair or Froyo have run unrooted/non-lag fixed on my phone for long since the experience used to be that bad. If they can close out the nasty bugs, Samsung may have a winner on their hands with this.
Since it is a beta, as mentioned earlier, the OS is still buggy in places. It can freeze up and a few reboots are the norm in a day. For me, that is worth living with since none of the cooked ROMs have given the kind of battery life this buggy release has given me. I have tried it in various combinations with wifi, GPS, data and it has consistently outperformed anything else I have run on this phone.
It will be really interesting if the guys at Cyanogen can fix up CM7 for the Galaxy S in time for Samsung's official release of Gingerbread. In either case, it is going to be finally happy times for SGS owners – another couple of months and the phone, which is an excellent hardware package, should get the kind of software it truly deserves, making it a really good iPhone competitor at a significantly cheaper price point.