We were there this past Feb and what a nightmare. I warned my dad about going then, but he insisted since he was meeting us from the Daytona 500. It just so happened that the race, President's Day and Mardi Gras all fell at the same time. Sooo WDW was bombarded with race fans, people off for Pres. Day, and lots of people from New Orleans (they also had off from school). Lines were awful, fastpasses were gone by lunch, if not sooner, and we hardly rode anything.
As for the weather...Feb can be iffy. Ive been before at Pres. Week and it was cool in the am and pm, and tee shirt weather during the day. This past Feb, it was really cold until the end of the week (when we left of course). I heard so many people talk about how unusually cold it was, and how others camping at Daytona froze.
If you want to avoid the crowds, go before Pres Week (a CM told me that they were pretty well booked up from then on due to Spring Break, etc). If you want to go during Mardi Gras, just be sure the race and Pres. Day doesnt fall at the same time.