I honestly felt bad leaving them to go to AT&T because they'd been so good to me over my 8 years with them. Sprint is one of the few companies that I recommend without a second's hesitation to people.
I used to feel the same way about Sprint, up until about 18 months ago. Their network (data) is severely overcrowded in many metro areas now. Before jumping to AT&T in April, the best 3G speeds I could manage in most places around Phoenix were around 500kbps. There were numerous times where download speeds would be <100kbps.
I have no clue how Sprint expects to be able to handle an influx of iPhone users, unless they expect that most of the adopters will be current customers that will just switch from their current Android phone over to the iPhone.
Definitely use the 14 day trial (I think Sprint recently eliminated the 30 day trial period) to test them out before doing any sort of number port.
If Sprint works, great. They really are not any cheaper any more, but they offer a lot more (lots of minutes, mobile to any mobile calling, free GPS Nav, Sprint TV...worthless, but free, 7pm nights) for the same amount of money.