I just traded in my Droid Eris for an iPhone. I had been reluctant to go with an iPhone because of the rumored horrible coverage by AT&T, but decided that the new towers they put up all over have solved the issue. My friend says he gets better coverage than he did on Verizon.
As far as the Eris: it's a nice phone, nice form factor, and has a good operating system including Sense UI. Here's the downside: The screen is too small (for me, my vision is getting worse), and it's too slow. Android has some really cool features, especially when you're integrating with Google. The browser is awesome and there are so many great free apps that it is nearly competing with the Apple App Store. But the Eris had a problem with the phone dialler interface that was just maddening. You'd click on something and have to wait 2-3 seconds for it to do anything. Trying to get on the browser takes almost 5 seconds just to load up. It seemed to me that the Android operating system is still a work in progress and requires way too much constant monitoring and tweaking to get it to do what I want it to do. I imagine that the Moto Droid is fast enough that you can get away with just letting programs eat up resources, but the Eris is too slow. I didn't want a Droid because it's big, ugly and heavy. I would have been interested in the Nexus One (really an HTC Passion) but couldn't wait for it to come out, and didn't like the T-Mobile plan they were offering. HTC generally makes good phones, but for a heavy user the Eris just isn't enough horsepower.
As for the iPhone: well I don't like GSM, it has radio noise, and the call quality is not nearly as good as it was on the Eris. The App Store is nice, but you can't filter on Free, so it seems like every crappy little app wants you to pay for it. However if you really want something, it's almost assuredly available for iPhone. This is a huge selling point--I can download those programs when people say "we've got an iPhone app for that."
The real reason why I like the 3GS is that it's really fast. The graphics processor is incredible--just check out google earth on it. Web pages load and rotate immediately. I have total integration with iTunes, which makes podcasting really easy. It also syncs up with my computer, not just Google. The soft keyboard is probably a wash with the Eris, but it's definitely good. Also there's a big hacking community for when I am ready to jailbreak it and use non-approved apps.
