I had the original, the 3G, 3Gs, i4, and my current one!
I tried switching when my i4 broke to no avail, was back on it a week later lol. The first one I got with contract, and I believe the 3G too, then ATT changed their 1-year policy to 2-years so I've been paying retail price for them :/ but it's worth it. No better phone out there! So convenient.