May is a very nice time to come, the weather is lovely, the restaurants are open. However do NOT go there if you wanna go for the clubbing, these will not be open, only certain clubs at weekend but it will not the summer line up with great dj on every night. Also the bars will not open and there won't really be that many clubbers around.
It all depends on what ur going for, if ur going for clubbing do not go in may!
I second Robo's views. If you want to come for the big clubs and DJ's and all the biggest events then don't bother going in May. The end of June is the earliest time to catch all of that.
But if you want to sample Ibiza at a more quiet time when the weather is as good as it is in the height of summer but there aren't the thousands of clubbers and beer boys etc, then May is a perfect time to go!
I would evensuggest go in Sept. Its not as crowded as the summer months, but there's definetly enough people to make it a great time.
I wouldn't go before Sept. 1st. The beaches aren't jammed, the lines to the clubs are smaller. The weather isn't quite as hot. There are plenty of hotties running around. Unbelievable!
Best times to go in my opinion is late June-mid July, and September. If you start your holiday from late July (e.g after the 20th) or anytime in August it is far too packed, and bloody expensive too and that's not just the more expensive booking fees!