I upgraded from a TracPhone to T-Mobile right before a business trip this past January. Two weeks after that, we used in in WDW and quite a bit, too.
I never encountered a dead spot. I looked at their coverage map before getting the plan and saw that most of the US Interstate system is in the coverage area.
While at WDW, I ended up keeping in touch with work quite a bit because of the blizzard, but never had a problem. Even better, the plan I got had 'text and type' and my phone was AOL instant messenger enabled (500 messages a month). With everyone I know stuck at home, my 15 hour drive home was much nicer being able to chat with everyone. DH usually wants to drive the whole trip, but he got jealous and I ended up driving so he could chat, too!
I've been so happy with the T Mobile that yesterday I upgraded to the family plan. Now we have free mobile to mobile with other T Mobile users (and a lot of our friends and family have it). We're also dumping our second phone line at home, because the 800 minutes in the plan is more than we would call long distance in a month.
Whichever plan you go for, make sure you find out what that company does if you refer people to them (not online, friends and family

) I got several coworkers sold on the T Mobile, then when I did the upgrade yesterday, I found out that I was eligible for a $25 credit on my cell bill for each person I refer to T Mobile!
Suzanne