We've stayed at the Nick Hotel quite a few times and have utilized the bus service on occasion. You do have to make a "reservation" to ride the buses so they know how full they are- I believe we only had to make the reservation going to the parks, not returning b/c sometimes you never know when you are going to want to leave. We have never been on one that had anyone standing. They give you a schedule of all returning buses so you can figure out which to take. The only blip we had was returning from MGM one night, we were dog-tired and the bus was 30 mins late. We also missed seeing Osborne Lights so we could be on that bus. Had we known the bus would be so late, we could have dragged our exhausted selves to see the lights, so I was a little annoyed. For the most part, it has been fine and I love that they have transportation to all Disney and Universal.
Have fun!
Lorraine