Hi Annie, the first 2 weeks in may are usually quite mild...not too hot! and probably best to take a cardigan or something to keep you warmer at night! It is quiet but you'll find it will get busier towards the end of the first 2 weeks!
We go in 3rd week of May, and apart from one day this year when it poured down we had sun all the time, fairly hot but comfortable to sunbath, evening were warm and did not need cardigan, going again 15th May 2010.
We went from 7 May for 1 week this year and the weather was fantastic, hotter then when we went in September around 35-40deg, no rain and it was quite busy, not packed but enough people about, the pool and sea are pretty cold and you will need a cardie or something to wrap up in for the evenings as it gets quite chilly, but I think its a lovely time of year to go