A little something to note about POP...
POP has nearly 3000 rooms with only 1 bus stop. You will almost certainly need to wait for a second or third bus if you are trying to travel at a busy time of day such as park opening.
The All Stars each have less than 2000 rooms and each has it's own bus stop.
As a PP noted there are very long bus lines if you are trying to leave the park at closing time or after fireworks, for example. However, EVERY resort is going to have a very long line at those times that will require waiting for another bus.
The best bus service I have had, to date, was at POP. I've stayed at BW, S/D, BC, WL, Poly, CR, POR, ASMusic (most multiple times)....best service was, without a doubt, Pop!! Yes, there are more people using that bus system. But Disney sends buses there more often. Let me share an interesting experience....
I was at Pop with my dh and dd. We got to the MK bus stop and waited for about 10 mins....as the bus pulled up, I realized I had forgotten something I really needed back in the room. I told dh and dd to hop on and I would take a later bus and meet them at MK. I ran back to our room, in the Rubicks cube bldg, far side by the lake/parking lot, got what I needed and back to the bus stop...walkking briskly. Well, as I got to the bus stop, a bus to MK was just coming in. It took about 8 mins for that second bus to get there!! DH and dd were just entering the park as I got there.
The Epcot resorts do tend to have less than stellar bus service. But, when you get to walk to two parks, you tend to forgive them that. I plan accordingly and leave a bit extra time when traveling to AK and MK from these resorts.