Up until this past Spring Break, we had been in 3 times in Jan, once in Feb, once in August and once early July (which, I assume has pretty similar weather to late June

). I can now officially say that I prefer Spring Break to Summer. I'm not saying I would never go in the summer, but I like Spring Break better. My first choice would be January, but since that isn't an option...
This is my take:
January - great time to go, low crowds, great rates. Didn't even need a touring plan ... got Crystal Palace Dinner ressies on same day. Taking your chances with the weather, but we had pretty good luck with low 70's about 75% of the time. Of course, it was raining and in the low 50's a couple of days too ... never saw the MK so absolutely empty on those cold, rainy Jan days (I would have hibernated at home in that kind of weather!). Never did make it to Typhoon Lagoon ... too cold for us, but did manage to swim a couple of times in the heated resort pools.
February: basically the same as January, but rates a little higher. Maybe a few more people.
July: Hot. Very Hot and Steamy. Rates were pretty good with our AP (reason we were enticed to visit, actually). Parks are pretty crowded ... didn't realize how many crazy people went to Florida in the summer!!! Managed to have a good time by planning well, but it was CRAZY HOT and MK was very busy the night we went for our CRT dinner - 140 minute wait for Peter Pan!!!. The heat is really draining. But, we got some great pool time. Rained nearly every day, but only for a shortwhile in the afternoon.
August: Hot. Very Hot and Steamy. Bascially same as July. Great rates and ease of last minute planning persuaded us to take this trip. Still had a good time and really enjoyed the heck out of Typhoon Lagoon and the resort pool.
April 2008: Spring Break. AWESOME!!! We spent 7 nights at WL and it was wonderful. We did not want to leave and DH immediately requested Spring Break off from his job for April of 2009 so we could return. The weather was perfect ... not too hot, not too cold. We also had the BEST transportation ever at WL (we have stayed there 4 times ... 3 in Jan, once for Spring Break) - I think they ramp up the number of buses and boats running during busy times b/c we rarely had to wait for a bus or a boat and there were a couple of times when we were the only family on the bus to MK.
I was worried as heck about Spring Break, but I did not want to take my kids out of school and did not want to go back in the heat of the summer. I read every Christmas and Spring Break thread I could find on the Disboards. Most people said, "Stay away, it is crowded!" I joined Tourguidemike and made a game plan according to his strategies and it worked beautifully.
I know that the parks were crowded when we were there (MK had 7:00am openings in anticipation of the hoards of people and Soarin' FP were gone around noon), but the crowds didn't really affect us with our TGM strategies. We saw a lot of upset, frustrated and weary spring breakers who probably hate WDW after their trips ... but I saw hoards of people walking
INTOthe MK at noon on a day when it opened at 7:00am.

The main pool at WL was also pretty crowded, but the quiet pool was pretty empty ... we swam every day.
If you are the type of family that can get up early and follow a plan, Spring Break is fine.
The biggest drawback to spring break is the pricing! EEE-GAADS!!! If you go, be prepared for high room rates and high airfare (book airfare early!). We are going again this year and I would love to stay at WL or Poly, but the rates were so high that we opted for Pop. It might be our last Spring Break trip simply due to the cost.