I just switched from an iPhone 3GS to a Captivate. My son-in-law and a co-worker both own Captivates, and had they not been out of stock last summer I'd have upgraded to it instead of an iPhone. Having said that, I was never displeased with my 3GS. Sure, it was pretty vanilla compared to what the Captivate can do, but I always felt restricted in what I could do. Everything looked the same, worked the same, etc.
When I got my Captivate, I was a little intimidated. It was almost infinitely customizable, and I like making things my own. Having said that, I was almost afraid to start monkeying with it. Also, I have Gmail and regular POP3 email, and I agree with most of the other posters that the native email app is worthless (not that Gmail is much better - almost the same interface). I did enjoy being able to check all four of my addresses in one app with the 3GS, but syncing contacts was a problem for me.
Having had it for over a week now, though, I can find things on the Captivate that I don't like, but things I do like a lot, like the widgets, the capability to use different launchers (I'm using Launcher Pro right now, and I've used GO Launcher. Might try ADW Launcher later) and even to put my frequently-called contacts on the homepage as shortcuts (even on the iPhone I still had to open the phone app and choose from favorites - even my ancient Samsung slider phone had a speed dial function). I also like the multiple home screens, but that's probably the most intimidating issue of all because I like to customize, and there's an infinitely blank slate waiting for me to fill it up.
I also like not having to use iTunes to load music or videos onto the Captivate, and my movies look much better than on the 3GS. Music playback is a bit of an issue, but Winamp has a feature that lets you control playback from the lock screen, much as the iPhone (with iOS 5.0) does. There are some apps not available for the Captivate and others I wish I had for the iPhone, and many are available for both, so I am not really missing a beat.
The jury's obviously still out on this one - after all, my priority is having a phone that works - but the music playback, web surfing, and 5mp/720p camera are nice to have, and I have more control over what's on my phone and where it's located, plus the ability to put a 32gb microSD card in it is a huge plus. But I'm not big on taking phones back just because it has some features I don't like. This one will probably stay in my pocket until it's time to refresh again.