We usually go over Memorial Day as our school lets out before most, and we can take advantage of "spring prices" and there usually aren't black out dates on the Power Pass then.
Crowds are moderately high...not as bad as summer crowds, not as bad as Disney at the same time, but busy.
We always stay at RP, so we have Express Passes. Most rides are 20-45 mins; depending on popularity...Minions I have seen around the 45-60 min window. Spider-Man, Transformers, Gringotts, Forbidden, Hogwarts Express are usually around 30-45 min waits.
All those times fluctuate of course, but I'm talking 10 am - 4 pm (peak in park times)
With Express Passes (on the rides that take them) we usually only wait 5-10 mins.
Weather-wise, we've had unbearably hot years (we swim more & do more water rides), we've had horrid thunderstorms & chilly weather after...it's really a crap shoot. We just go with the flow & have a blast. The worst day at Universal beats any day at home doing dishes!
