Since our cell phone contract with Cingular expired a little over a year ago, we've just been keeping the same plan on a month to month basis. Three out of the three Sony Ericson T610 phones we had were broken to various degrees. We didn't have any coverage problems with Cingular (AT&T Wireless), but they no longer offered the same plan, and we would have been paying more for fewer minutes. I ended up getting a great plan with Sprint with unlimited data (thanks Mike for telling me about it last summer), and I bought a Treo 700p to make good use of it. I've enjoyed it so far (in fact, I'm typing this on my Treo right now), and it's nice actually being able to check my email every day.
Heather doesn't use her phone that much, but I wanted her to have one when she's driving, so we ended up getting a nice prepaid plan from T-Mobile. A 1,000 minute prepaid card is $100 and lasts for a year, so it comes out to a little more than $8/month. Not bad at all. We're paying less for more, which I don't mind one bit.