Just went for a week at the end of October, and all four adults in our party used cell phones.
I have Cingular service and found that (for the most part) it worked great, especially when outside (as mentioned in another post). There were a couple of "dead spots," like inside The Land at Epcot (no reception at all when standing in line for Soarin'). I was very happy with our ability to communicate - we called each other 5 or 6 times a day, since we often separated to do different things. First time I had tried to do that, and I thought it worked great!
(One tip, though - it's hard to remember to change your ring feature to "vibrate" when on a quieter attraction - but important to be courteous to others. I also kept forgetting to turn the ringer back on after having the phone on vibrate, and missed a couple of calls. Oh, well, still totally worth it to use cell phones!)