I got to ride it the first weekend and ended up riding it 5-6x (via a combo of FPs, standby, gifted VIP tour). It really is a fun, happy ride. And each car gets a different experience because sometimes things will be happening before you get into a room or continue after you leave, so each one gets to see something slightly different. Also there is just so much going on, you can always catch something new.
The song is really infectious and, as I mentioned, it really is just a delightful experience. I do agree RotR is more technologically intense, but MMRR is still a great addition to the park and I was really happy I got to experience it pre-shutdown!