We got to iPhone 3GS phones in the last month. I've had mine for about a month now and my partner has had hers for about 2 weeks. I transitioned from a Blackberry and was very concerned about getting all my info over and becoming familiar with the iPhone interface. I used GoogleSync to transfer over all my contacts and calendar items from the BB to the iPhone. It worked FLAWLESSLY. I was very impressed! No info got lost and I just had to make a few quick edits to a handful of my 200+ contacts. I immediately started loading apps; I made the mistake of loading on a few apps via the phone's OTA application. This resulted in all the apps ceasing to work suddenly one day. I loaded them back on through iTunes and have not had a problem since. All the apps work well (I'm using Facebook, Brushes, BeeJive, Maifesto, Remember the Milk, Sirius, DocsToGo, BofA, Tweetie, and a Washington Metro app).
I have been incredibly impressed with the speed and capabilities of the iPhone. I loved the email functionality of the Blackberry, but I don't feel as though I've lost any email functionality with the iPhone. The BEST part about email on the iPhone is that I have access to all my folders; this is especially important for me because I use lots of folders for both my Yahoo and Gmail accounts.
My partner's iPhone did have an issue. About two weeks after picking it up, her screen started randomly shutting off. Yesterday morning, we woke up to find her phone completely dead. They did a hard reset at the AT&T store, but agreed that something was wrong with it (the screen would just suddenly shut off). Thankfully, the AT&T store replaced it with a brand new phone that day. They were great; they got everything set back up again and replaced her screen protector for free.
Overall, the iPhone experience for us has been a positive one. AT&T's service is not cheap, but I've been very impressed with their service overall.
The only downsides to the iPhone I've experienced thus far is the less-than-stellar battery life and the inability to get apps through any other source than iTunes. Regarding the battery life, I recognize that Apple probably made a conscientious decision to use a less powerful battery but keep the phone smaller. For me, it's not that big of a deal. I just make sure to always have my charger with me and I got one of those RichardSolo extended battery packs (which is really nice; it's small and works great). The app issue isn't really a deal breaker for me, although it would be nice if some of the apps were a bit cheaper.