I spent a week there in june/july 2011 while I was working in Tanzania and loved it. We stayed in Paje at one of the many hotels near the kite beach. If I did it again I would either stay at Paje by kite, which had some awesome parties, or the accommodation at the Zanzibar kite centre if its completed (literally next door to Paje by Kite).
Conditions were fantastic, from memory we had wind just about every day. Most days it was between 15 - 20 knots from about 11am through to sunset. Really nice butter flat water at low tide, just have to look out for the sticks the locals catch seaweed on. Choppy at high tide with a few waves to play on out the back. Great setup at the kite centre with hammocks, couches and good people. Fantastic food at the restaurants on the beach and excellent local beer. Heaps to see and do around the island if the wind isn't blowing. The scenery is also postcard perfect.