We went the first week of May this year - it was FABULOUS! Weather was warm enough for us (being Canadian we are used to cold) - mid 70's most days - a couple afternoons in the 80's. Kids needed jackets a few evenings, but I got by with shorts and tank tops every day!
As for crowds, the weekdays were great - almost everything was walk on first thing in the morning - we had no concept of crowds till Friday morning - it was an eye opener!! We skipped Saturday and went again on Sunday, which was still busy, but probably not as bad as Sat. We used the FP system whenever we could. I think our longest wait for a ride was about 15 minutes for Astro Orbiter, later in the morning. We did FL on a non MM day first thing in the morning, and the only real wait was for PP - about 7 minutes. Dumbo we waited one round, and many of the other rides were walk ons, even later in the morning. I'd definitely go in off season again!
ETA - d'oh! I see you are Northerners too, therefore weather shouldn't be much of an issue for you either! Guess I should pay attention to where people are from, since the terms "warm weather" are so subjective! We still had snow on the ground when we got home, so to us the weather in Cali was pretty darn nice!!