2 Years ago in our Dec trip we had Cingular phones and I brought my professional Motorola 2 ways.
The phones had very spotty coverage, the 2 ways worked very good, they were just bigger than I liked at Disney. I used my own licensed frequencies on direct and had no problems. The other family that was with us had FRS radios and while we could talk to each other (I programmed my radio to talk to theirs) so could everybody else and there was constant noise on their frequencies.
The last trips we have taken, we had switched to Sprint with Ready Link (Sprints nextel type 2 way service), left the big radio's home and the phones worked great. Great coverage and very convenient. I have not found a dead spot yet (except in a couple of attractions).
The problem with the FRS radios is the power and the fact that everyone has them. The GMRS radios are better as they have more power and private line capabilities, but require a $75 license (good for 5 years).
If I did not have 5 watt radios to use, I would most likely stick to the cell phones (as long as you have a nationwide plan)
Of course, If you have younger kids without cell phones and you allow them their own space, you would most likely need radios despite the shortcommings mentioned. Some communication is better than no communication.