I for one am glad you shared, I think that this is a place to do it, and you are probably not 100% convinced of your decision, which is why you posted it here.
I'll tell you what I do and do not like about my captivate compared to my 3gs, maybe it will help you with your decision.
The 3gs, yes, the 3gs, not even the iphone 4, has SIGNIFICANTLY less lag than the Captivate. Those who disagree haven't owned both. Opening up applications, web browsing, switching around the homescreens, everything is less snappy. This is my biggest complaint so far and it is a big one for me. To alleviate the problem I have installed launcherpro and adw launcher. Launcherpro seems to help a little. I have also installed SetPCU and keep the processor running at 1ghz, I have chargers everywhere so this isn't a problem for me, and this seems to help a little as well. I have also rooted my phone and deleted the ATT bloatware. I have not yet installed any of the roms that address lag as I am awaiting for a streamlined procedure since this is all new to me and I am still timid about bricking my phone. But I am certain that I will install many other updates as well.
Though it may be seen as a pain in the ass to some, I thoroughly enjoy tinkering with my phone. The development community for Android is robust and usually helpful. The fanboys are not nearly as brainwashed and annoying, typically willing to admit faults as opposed to getting defensive, something that is pretty much non-existent in iphone forums. Ultimately, the customization of Android phones is what has won me over. My phone feels like mine, not something I leased from big brother Jobs. Yes, I kept up with jailbreaking my iphone, but once you get into it, the customization level just isn't there with the iphone. LauncherPro and ADW launcher are just a couple of prime examples, you could never do something like that with Apple products. Some would say that they don't have to so they are ok with it, I like having the option.
The GPS issue is annoying, mine works ok, enough to show me within the vicinity of where I am, but not good enough for a modern smartphone. If this is not fixed before my 30 days are up, I am likely going to return my phone. I am an attorney by the way and I think that this is class-action material. You cannot sell a phone advertised with a working GPS and then release this garbage. Very upsetting. However, I honestly believe that this will be fixed, and relatively soon, so I do not think I will have to return my phone.
I'm also a little dissapointed in the camera, but it is livable.
I hope that you are happy with whatever decision you end up making. I know I am, I cannot imagine going back to Apple, and I am stuck on ATT because of a family plan. I say take the Android for a spin, give yourself a couple of weeks to become acclimated, install some things to speed it up a bit, if you don't like it, then return it and go back to Apple, but if you give it a sincere shot, I doubt that will happen.
Hope this helps.