I switched from an iPhone 4 to the TB, and switched back due to a few frustrations. However, after doing more research, I am going to switch back to the TB (haha). I've owned an iPhone since the 3G, and I hadn't left the iPhone community until I bought the TB. I'll tell you why I switched, and why I'm switching back.
I'm not a fan of HTC's replacement contacts application. It looks like garbage, especially considering the stock application is so good. I have a Mac, and I love managing my contacts and calendars in Address Book and iCal—I'm not a huge fan of using webapps to manage all of my information. HTC's keyboard is HORRID, IMO. And keyboards downloaded from the Market leave something to be desired. (For instance the Gingerbread keyboard, which I like, works, but it isn't terribly responsive--remember, I'm coming from an iPhone 4.) Also, some things you think should happen (like music pausing when playing a YouTube video) just don't. Finally, the power button just isn't very good.
Now, why I'm going back (there are a lot of reasons). I like being able to quickly search with the press of a button. I actually really like having a little notification light. The notification system is drastically improved. Google Voice and texting with Google Voice just works. The problems I cited above could be remedied with root and a new ROM (waiting, first and foremost for CM7). Amazon Cloud Player is pretty exciting. I've found some decent syncing solutions that will keep my groups and calendars in sync with Google. Finally, I'm tired of Apple thinking I'm a moron. I'll miss the hardware, but I'll be happy to be back on a platform that I can do more with.