I haven't stayed at ASMo, but my dh and I stayed at ASMu for 5 nights this past January. I'll try to cover things that apply to all the All Stars. Hope it helps in your decision.
The good points: A truly wonderful mousekeeper, a room that was just fine for 2 people, excellent bus transportation, a great price.
The medium points: The wait at check-in was about an hour, which seemed excessive. The food court food was adequate, but hardly exceptional, and the food court was basically closed late in the evening. Our room was in the building farthest away from the food court/bus stop and even though we're used to walking everywhere (we live in NYC), after a long day in the parks, the walk back to the room was just too much--which may have felt this way because of the bad points, below.
The bad points: I will preface this by saying that dh and I do not wake up early on vacation (even at WDW!). We tend to sleep until at least 8:30 or 9 am and sometimes later. But at ASMu, we were unable to sleep past 7 am, sometimes earlier. The noise from the landings was just too loud. Noise in general inside the room we were in (we were on the 2nd floor--so had rooms on all sides of us) was, well, noisy. I'm not a light sleeper, but I was unable to get an actual night's sleep the entire 5 nights we were there. This was not a case of being in the midst of a cheerleading or football group (there were none there during our stay -- but, be careful, these groups do stay at the All Stars and I've heard they can be boisterous), but just the way the resort is built. The walls are not thick, there's a cement walkway right outside one's front door, and as I'm sure you've heard, the sound from neighboring flushing toilets is HUGE. This can be slightly alleviated by leaving your own bathroom door shut. However, there's no way to tamp down the sound of people on the walkways, of babies crying in the rooms, of people talking above a whisper, of not-even-that-loud neighboring tv sets, etc. Needless to say, this was the killer for me and we won't be staying at an All Star resort again, regardless of the great price.
I'd also like to add that if you're looking for a beautiful room, ASMu fails the aesthetic sweepstakes. I haven't seen the inside of the rooms as ASMo, so they might be nicer. Even though our room was not that attractive (or even adorable), this didn't really bother me...but I had too much time to think about it while I was lying in bed awake, so this developed a significance it wouldn't have otherwise had!
I hope all this helped. If you wake up early, the A.M. sounds won't bother you, and if you get a room on the top floor, you'll probably avoid at least 25% of the room-to-room noise that we experienced. I know a lot of posters say the All Stars seem quiet to them--it may depend on room location, but I don't really know. It also may be that the type of noise that kept me awake doesn't faze you. This is all just my experience--not the definitive word on the All Stars!
But whatever you decide, you'll have a great trip--you'll be at WDW!
--miffy--