I'm currently using an HTC Incredible II and will definitely be upgrading to the iPhone 5. In all honesty, this has been a dependable little phone and never gave me any problems until I did the last update. Now sometimes random letters will stick when I'm trying type something. But other than that I'd have to say it's been the best Droid I've had so far.
My reason for switching is because I'm just tired of Android. Sure it has lots of positive features, but I'm just tired of the messy, unorganized aspect of it. And as I mentioned earlier, my current phone is the only Droid that hasn't been a pretty big pain in the a** as far as malfunctions and not working properly goes. I've had problems with all the others. I went through four months and eight Thunderbolts when I did my last upgrade - all that had the same major problems that left me unable to use them like I should have been able to. I finally told them I wanted to be put into a new model, and this Incredible II was what I got. I look forward to the iPhone being a lot more dependable.
In addition to that, I've always been impressed by the iPhone's camera. I take a lot of pictures on my phone so that's a big plus for me. There also seems to be a lot of apps I'd love, so that will be nice. And I've always loved the overall appearance and form factor of the iPhone. I realize that will be changing a bit with the new one, but I also realize that I can't expect it to never change. Compared to other phones out there that drastically change with each reincarnation, the iPhone still manages to look recognizable.
So that's about it! I'm super excited for it to be released and will be pre-ordering through Verizon as soon as they begin accepting them!