I think they use Sprint towers and they have good coverage where I live. I will say that sometimes when connecting a phone call on wifi, if my connection isn't great, it takes a long time to actually hear the other person once the call connects (sometimes as much as 10 seconds). This has been annoying. I've also had calls drop when walking out of wifi range as it switches over to the cell tower. But honestly, for $10/month these are issues I can deal with.

I have the Moto-X 2nd gen and love it, it's been a great phone. I'd probably go with the Moto-G before the Moto-E if I didn't pick the Moto-X. It's not much more expensive than the Moto-E but it's a quite a bit better if I remember correctly. Also, I heard from a buddy that the Moto-X 3rd gen may be coming out soon but I haven't confirmed this myself.