My boyfriend sells cell phones now, so that's all I ever hear about from him. Here's what I've learned from him (when I'm actually paying attention to him while he's talking about cell phones
):
Sprint is crappy because they're doing that "building from the ground up" thing, so they don't have as many towers as many of the other companies and they can't use other companies' towers. So, unless you're near a Sprint tower, you're not going to get reception. Sprint will be better in a few years when they build more towers, but until then, you should probably go with something else. All of the other large companies (AT&T, Cingular, Verizon, VoiceStream/T-Mobil) can share towers so they get better reception in most places than Sprint does.
I personally have Cingular and I've never had a problem with them. They also usually have really good plans (i.e., nationwide service w/ no roaming and now they have rollover minutes). My boyfriend is an AT&T customer and that's also who he works for, so he's brainwashed me into being pro-AT&T and anti-Verizon.
I know people who have both of them and they like them and dislike them for different reasons. Everyone I know that has VoiceStream really doesn't like it, but it might be different now that they changed their name.
That's my non-expert opinion...