We've been 3 years in March ('02, '04, '06) for about 10 - 12 days each and love it because the weather is gorgeous. Only once in all those days has it rained. We typically pack a light windbreaker and a pair of jeans just in case. This year a cool snap came through and we switched our AK & BB days, but we still wore shorts. We went to BB once and TL twice and swam all the other days. Although I believe '05 had quite a bit of colder weather.
Like others have mentioned, the crowds are manageable, and there's typically a cheerleading competition at MGM around the 12th, but we still went on that day and didn't have any problems. We typically go the 2nd week and I think it keeps getting a little more crowded throughout the month. I agree that if you have a plan, you shouldn't have any problems.
One thing we've had trouble with is the blooming trees. My DH has severe allergies and has learned to get a kenalog shot before we go and that helps.