I really hate to say it, but.. First, I have never used an iPhone. My last phone was a Blackberry Pearl, so I don't really know, but from what I've heard the iOS is pretty nice.. I'm a PC user, though. I thought a Linux OS would be right up my alley. And it is.
I now have a Droid. Unfortunately, while simultaneously being the most awesome phone I've ever used, it is also the worst. It pains me to say that because I generally hold Google in such high regard. It does things that no phone should do, like spontaneously delete texts. That is probably the single most aggravating thing I've ever encountered with any phone ever. It doesn't help that my first phone was defective and would crash every 20 minutes.
I have never encountered so many headaches with a phone. I really love it the idea of it, but it was poorly executed, IMO. It just could be so much better...
It's a little slow. Or it can get that way, at least, if you don't manage your tasks. Battery life is very poor, even with task management. With even heavy texting(as opposed to browsing the web), the battery is toast in as little as 6 hours. It sucks not being able to get through the day without having to charge your phone. It does fucking retarded things like pick the wrong person to reply to when you're texting multiple people and moving fast, even though you see the correct person's name turn orange. When you go back, the correct person's name is still selected, even though it put you into a completely random(as far as I can tell) thread. It's really bizarre, and extremely frustrating.
If the iPhone doesn't do shit like this, it's no wonder it's so popular. And it will probably remain that way.