I've been to Phuket a couple of times during the mid-year moonsoon. I remember the first time, when I saw Patong Beach and there were warning signs up on the beach, but I was delighted because there were some decent waves and I could body surf. It rains, it stops raining, the sun comes out, later on it rains again. I think the other monsoon later in the year (around Oct/Nov) is more serious, that's when there is flooding and roads closed in the South. But with the mid-year monsoon you can be lucky and still have nice weather.