Allow me to throw my two cents in. I got my Captivate on Tuesday (1/17) and updated to GB yesterday. So far, I have only had one problem - Wi-Fi connectivity seemed to break when I updated. After the update, the Wi-Fi connection spent an inordinate amount of time trying to get an IP address from my router. I switched to static IP and assigned an address and got connected, then later I unplugged my router and plugged it back in, and switched back to DHCP and got a connection. (Interestingly, it was the same address I had assigned through static IP.)
Other than that, I have had no major problems with GB yet. I like the new icons in the notification area - much cleaner than the ones in Froyo. None of my data disappeared, although some of my settings did. It seems snappy enough, but I am comparing it to both my old iPhone and a few days of using Froyo, so I perhaps don't have enough data to make a valid comparison.
I'm definitely an Android n00b, but I find a lot of things I like about it that are different from iOS, most significantly the ability to customize. I don't like the lack of ability to "multitask" (in iOS, you hold down the "home" button and all of your open apps appear at the bottom), but the "Recent Apps" window is somewhat helpful. I wish it showed more than just the last six apps, though. Once I figured out how to open new windows in the browser, that made the transition from iOS a little easier. And the ability to set up direct dial shortcuts on the home screen is a real plus - I don't have to hit "phone," then navigate to "favorites" to get my frequently dialed numbers (like my wife).
And Samsung has always put out quality products, so there wasn't very much trepidation about switching to the Captivate. Overall, I'm fairly pleased so far, and once I figure out how I want to configure my home screens to be more productive, I'm sure I'll like it even more.