I've stayed at ASMu (96), Sports (04 & 06) & most recently, Pop (8/07). I was really excited to try Pop, as so many people on the boards rave about it being the best value. I was extremely disappointed. For my group (Mom 63, DS 7, DS 2 & DD 4 Mos), the size of Pop was a problem. We drove to WDW & actually ended up using the car to go up to the main building from our room in 90's. Also, having 4 floors (we were on the 4th) made it more difficult for us. 4 floors is just to much to walk up with 3 tired kids and the elevators were incredibly slow and not air-conditioned. There's also no easy way to get from the parking lot to the elevators as they're in the center of the building and you can't cut straight across because of the landscaping.
We didn't use the bus service, so I can't comment on that.
Personally, I just found the All-Stars, especially Sports, easier to get around and the food court much less crowded. Also, we had requested 1st floor near parking and they were unable to accomodate us at Pop. I've never had a problem with requests at AS and they've even found us a different room when I mentioned my request. Maybe the fact that Pop is so much more popular has an effect on that.
Just thought I'd put my thoughts in, since so often all the comments on Pop are overwhemingly positive and it's nearly always recommended over AS.