Well, it's been just over a week since I got my Samsung Captivate (which is selling for $50 right now!) and I figured I'd share my experiences switching to it from my old iPhone 3G. Sadly, I have to say it's still a bit rough around the edges. I've had a number of problems with it, but none of them have been insurmountable.
Even though I'm still on Android 2.1 (FroYo, aka 2.2, won't be pushed on my phone until September or so), I've had no problems at all with performance. Everything starts up pretty quickly for the most part (occasional lag spikes, but I got those on my old iPhone as well). The 2.2 update will supposedly make my phone even faster, so I'm excited about that.
All the apps I cared about are available in the Android Market (things like a twitter client, Amazon shopping app, Google Maps (duh), Fandango and a handful of others). I was also able to find apps like Adobe Reader and Quickoffice for loading pdfs and MS Office documents (which is handy and something that I never found on iPhone).
Likely due to a higher-resolution screen, the YouTube video quality on my Samsung Captivate also exceeds that of my old iPhone 3G. Not sure how it compares to the iPhone 4.
What I really like about the Android is that editing my Google Contacts auto-updates my phone within seconds of making the changes which is really nice. I didn't pay for the MobileMe service for iPhone (which supposedly adds this feature), so it's a nice bonus that has saved me a bit of time and trouble already.
While the music player on Android isn't bad (it's quite usable), it lacks some finishing touches that Apple put into theirs. For example, iPhone's music player remembers the most recent audio track and position you were listening to when you launch it. No amount of browsing your music collection will confuse it. This is not true with Android's. A "Go back 30 seconds" button would also be a really nice addition to Android's music player that iPhone has and I've found to be quite useful over the past 2 years.
When making a call, the iPhone's display is much much nicer than Android's and the Contacts app itself is more intuitive. I don't even understand what most of the tabs are in Android's Contacts app, for instance.
The following 2 problems are the absolute worst usability problems I have encountered, and they are pretty bad.
It took me a while to figure this out, but when you plug your Android phone into your PC, the PC won't see your Android device over USB Mass Storage at all until you open up your phone's "Notifications" area and tap on the notification saying something about connecting via USB. Once you do that, it opens up a dialog with the option to mount the drives. Only after you take these steps does the Android device show up as a USB Mass Storage device on your PC. This is just awful. Why are these manual steps even needed at all? If I have edited my phone's settings and selected "USB Mass Storage", it should just assume that's what I want to do when I plug it into a PC. There's no good reason for it to make me manually go through those steps. It also seems I'm not the first to be confused by this as there are a number of users complaining about this on various Android forums. From what I've seen, things have gotten better in Android 2.2 (it no longer makes you navigate to the "Notifications" area, but it still requires you to interact with a Mount/Cancel dialog).
The above photograph is what happened when I tried to place a call to about half the people in my contacts list. Yea, that's right, it crashed. After a bit of fiddling, I was able to figure out what it was about those contacts which caused the problem and submitted a bug report. Luckily, the workaround that I found was trivial and so I just loaded a web browser and edited the Birthday field info for those contacts and waited a second or two for the Android to auto-sync. Had I not been able to find a solution to this problem, I would have returned my phone and gone back to iPhone.
Android has a lot of room for improvement (buggy, unpolished), but if you are a Linux Desktop user like myself, you'll probably feel right at home.