Actually, we found the busses to be very efficient. We saw Illuminations several times during our Christmas trip and always took the bus back to WL.
A few times the bus was waiting for us when we walked up, but we never waited more than a few minutes for one. The parks were extremely crowded, but the WL bus was great!
We do take our time leaving the park. We've found out that if you just sit tight for 5 minutes, the crowd thins out considerably and it's a nice stroll through Epcot!