I got the original iPhone (2G) the week before the 3G came out, when the O2 shops were clearing them out for £159, to make way for the 3g.
I stuck with it until the 3GS came out and I bought a friend's 3G as he was upgrading to a 3GS.
I stuck with that until the i4, which I got on release day and I bagged a free 4S in May this year for converting my 1 month rolling SIM Only contract to a 2 year contract.
I'm with 3 and had no plans to leave due to the unlimited data, free tethering and 2000 minutes allowance, so I thought I might as well take advantage of the free 4S and lock in.
I had it unlocked by 3 immediately and will be selling it and getting an i5, again SIM Free and unlocked from Apple.