I left Sprint for AT&T in April. Sprint just didn't work at my house. That is a specific issue though, and can happen with any carrier. I had been dealing with it because they offered much more for the same price as everyone else.
However, at the time I left Sprint, their 3G data speeds were atrocious (56k modem speeds) in many places around Phoenix. If you go to sprintusers.com, you will see similar reports from around the country.
Likewise, there are other places where Sprint seems to have decent backhaul and no data issues (SoCal, Chicago).
One thing I hated about Sprint was that if one phone needed data and you were on a family plan, all your lines had to have data. With 4 lines on my account, I only needed it on 2 of them, but was paying for it on all 4. I rationalized it by saying that even paying for all that, it was the same price as AT&T and a few bucks less than Verizon, and Sprint gives you free GPS nav too.
But, with the service degradation and Sprint's new imposition of the $10/line smartphone "fee", I decided to switch and have been much happier with AT&T. I get half the minutes, no free NAV, and no more data on two lines for $5 less that what I was paying to Sprint (so I lost a lot of features to save almost nothing), but for me the service works better.
I suggest using the 30 day trial and then porting a number later if you are thinking of switching.