Coverage on all the carriers is far from perfect. If you live in a place with sucky coverage from your carrier, ask a friend using another carrier to come over and see if his phone works there. If it does, switch carriers.
For me, in my locations, Verizon has been great.
When testing coverage we'd get loaner cellphones from prospective carriers to try it out ourselves.
The wife and I have been traveling, and my Alltel phone was perfect
whereas her T-Moblie was.... "no service"