I didn't buy the 3G. When I got the 3GS I gave the wife the 2G and we're on a family plan. She's eligible to buy again in November from her old sony/ericsson, so as long as nothing busts or gets lost/stolen I could get a subsidized phone each summer if I wanted.
The big reason I didn't get a 3G was because we didn't have 3G service here then and didn't want to pay an extra $15/mo for nothing. Plus we had a baby on the way. And also I was still sore from paying $600. If that situation was all different, I probably would have, so I could see myself certainly upgrading each summer.