I converted from the Palm Pre when the Iphone 4 came out. Here are my thoughts;
Pros;
LOVE the display. The resolution is amazing.
Love the form factor. The weight, look, the whole package. I wish the camera lense were a little more scratch resistant, but other than that (and the obvious antenna issue), there is nothing I would change about the design of the phone.
Apps. Oh, my god. I've literally spent more on apps already than I did for the phone. It's rediculous. But rediculous in a good way.
Cons;
Have to jailbreak to do anything I want to do. Coming from completly open source palm, this is a bit of a bummer. Kind of a tradeoff though, because jailbreaking is easier than the homebrew on palm, and you start out with more on the iphone, so there's less far to go.
The OS took some getting used to. Palm got it absolutely right with the card thing on WebOS. iOS is a tiny bit clunky in comparison. It flows well once you get used to it, but it is just a little less intuitave. It's the difference between a z06 Corvette (a very fast car), and a Bugatti Veryon (an extremely fast car). Both are awesome, but one more so than the other. With that said, the Iphone 4's hardware makes up for it's deficiencies. But on equal hardware, palm would have the edge here.
The antenna issue. I LOVE the way my phone looks naked. But I had to order a bumper, because I can't constantly concentrate on how I'm holding my phone. I'm left handed, so my finger shouldnt ever touch the kill switch, but one way or another, it regularly does.
Overall, I love this phone. I would buy it again in a heartbeat. I'm absolutely not complaining, as every phone has flaws. This on is about as close to perfect as I have ever seen.