We bought walkie talkies for our last trip- they worked very well, no interference and came in quite handy during the trip. We used them quite a bit as our group managed to split up quite often while in WDW. We have even used them quite a bit since our last trip- at fairs, malls, etc...
We got Motorolas w/ a 2mi range, I think if you get a 5mi range you need to register w/ the FCC (but I could be wrong), our walkie talkies have several channels, along w/ privacy channels, so we never had any interference or mixed signals.
I think I ordered ours through circuitcity.com and I think that web site also has an overview of how they work etc. I think the motorola website has info also.