I'm going from a Captivate to an IPhone4, the following pushed me over the edge.
1. Samsung's lack of PR skills and updates regarding their phone and the issues it currently has.
2. After last week's update failed to resolve the 3 month old GPS issue, decided to call AT&T and Samsung. Samsung tech support was abysmal and got a different answer each time. AT&T was helpful but you can clearly tell, like me, they were at the mercy of what Samsung was pushing out the door. Yes, I know these companies work behind the scenes and there is a ton of pull. If that was the case, the GPS issue should have been solved sooner. At the end, I felt like there was no light at the end of the tunnel with regards to this issue.
3. A lot of apps tie into the GPS functionality, so to say that is not a concern is fooling yourself. Google Nav, which is free, relies on this functionality as well as anything else related to Google Maps, Places, etc.
4. Last week's update introduced a USB/Wifi bug that was annoying me all weekend. Whenever I used the USB connection to copy files, Wifi would not work. A simple reboot would resolve the issue. The other issue that bothered me with the update was that some other changes were sneaked in without them being advertised. These changes caused issues for some of us and removed functionality at the same time (USB Tethering).
5. The Galaxy S line has had a broken GPS since it was originally launched then made its way to EU and USA. How can Samsung not have an update for this almost 6 months later is puzzling, possible hardware issues, who knows. Vibrant got an update this week, forums already filling up with GPS complaints as well.
6. Froyo, no clear indication of when they would make it available. I blame this on their PR skills, they do a poor job of communicating and keeping their user community informed.
7. In-Car USB does not work, you have to buy a 3.5 mm cable in order to play MP3s in your car using the phone.
The display is awesome, the phone is smooth for the most part with some minor lag. More and more apps are coming to Android, especially games, so that's another plus. I would recommend Android without even looking back but I think Samsung is the wrong company to deliver the experience. Look into Evo, Droid X or any other top rated Android phone.