iOS4... everything just works seamlessly.
I made the switch from Blackberry OS5. Just a few gripes I had with that:
-SLOW. There was no use in putting apps on my BB because they would a. take forever to load, b. not load at all and/or c. run incredibly slowly. There's literally no fun factor to this phone. It's utilitarian in its approach and its use. I shouldn't have to pay $100+ a month for a phone that I don't even enjoy using.
-Reboot time. It would take on average 10 minutes for the phone to start up when I had to do a battery pull because the phone would decide to freeze. Battery pulls seemed to be a constant in the last couple of months I owned it and I'm not sure why since I didn't have any applications or excess languages on it.
-Browser experience. Wow. You know a browser is bad when Google search takes more than 2 minutes to load. I would actually wait until I was around my desktop if I had to search something. The browser on OS 5 is failure. I see that OS 6 has marked improvements but ehh, too late for RIM.
-Truncated e-mail messages! This was annoying since my job sends around large .pdfs and excel spreadsheets. What's the point of advertising your e-mail services if you can't actually view the e-mail? Yes. They would arrive pretty quickly but it's useless if I have to wait to open them at a computer. I use exchange for my work email address and have noticed that the arrival time on my iPhone is pretty damn comparable to my BB.
-BBM is annoying. I actually much prefer my messages not be seen as read or delivered. That's a personal issue and I'm sure everyone who uses BBM loves it but people seem to have this urgency when a D turns to an R.
-The OS is filled with menus behind menus on top of menus beside menus near menus just to get to things. I'm not sure how RIM is going to handle this in OS 6 but it shouldn't take me 3-4 menus to get to a simple setting in OS 5.