We've been to Disney twice during the first week in March, and found crowds to be fairly low each time. It did get much more crowded when that first weekend hit, though.
As for weather, it can be hit or miss. For our first trip, it was downright cold for the first half of our trip. I didn't feel like we had brought along enough warm clothing. It did warm up for the second half. We only got one day when it was warm enough to not freeze in the pool. When we went in '04, the weather was absolutely perfect!!!!
Overall, it's a great time to go. I'm a bit nervous about breaking our trend and going in October this time.