I actually just made the switch from an iPhone 3GS to my first Android phone (the MyTouch 4G on T-Mobile). The biggest reason I made the switch was because I wanted to save money (I get unlimited texts, data, and voice for $40 less per month than a more limited plan on AT&T). So if you're contract is up, definitely take the time to look for other carriers (assuming you're running your 3GS on AT&T).
As to the phone itself, I have zero regrets about making the switch. Here's the benefits I see:
1) I'm a heavy Google user (gmail, calendar, etc) and the Android phones make this super easy. All I had to do was enter my Gmail accounts into the phone and all of my mail, appointments, and contacts instantly transferred over.
2) T-Mobile, with the $80/mo unlimited everything plan, allows you to get free Wi-Fi tethering of my phone to my laptop. Very nice.
3) Every app (with the exception of Scramble) that I used on my iPhone is available through the Android market.
4) The Navigation app (not Maps but the actual, free Navigation app included with the phone) is as good as any Garmin I've used. It's awesome if you do a lot of driving and frequently need directions.
5) The phone and software is super snappy, clean, and fairly intuitive to use. Front & back facing camera, flash, video, high resolution screen, etc.
The only two disadvantages to the Android phones are:
1) It's not quite as seamless and easy for syncing data. The iPhone you just plug into your computer and you're syncing your music, podcasts, photos, etc. It's a little more effort (but not much) to get the Android phones to sync this kind of data. Also, the software isn't quite as slick.
2) You won't have as many accessories out there to buy. If you're big on lots of gadgets for your phone, you won't have nearly the selection of cases, extended batteries, and other gizmos you'll have available for an iPhone.
Overall, my suggestion would be, if your contract is up, find the carrier out there you want to use and then get one of their top-end Android phones. If you want to stay on AT&T, definitely check out the Atrix - that phone is fantastic. Verizon has a ton of great Android phones right now (the Droid X has the best on screen keyboard I've ever played with). And, as I just mentioned, I completely love my new MyTouch 4G on T-Mobile.