Just been there at the end of October. It was right at the end of the season so didn't get much time on the water but it was awesome when I did. Make sure you pick the right time which probably would be August/ September.
Super smooth warm water/winds, and only a couple of other kites to share it with! I wish I could go back there.
We stayed at Bellevue Zanzibar in Paje which is on the East Coast. The Bellevue is a guesthouse owned by a Dutch couple who are very nice. It was very nice and intimate place to stay for a good price. There is no nitelife there though. Dim who is one of the managers, owns Kitecenter on Paje beach. He does lessons and gear hire and will drive you to the kite beach. Check out http://www.kitecentrezanzibar.com
Also while your up there ask about the restaurant on the rock island. Can't remeber what it is called but absolutely amazing spot if you don't mind spending a little dosh for lunch.
We also stayed is Ras Nungwi (North point) for a few days but no kiting there however very nice place to stay.
Safety, like any African country as long as you have your wits about you there will be no problems. Take care at night in Stonetown. We ran into some drunk that wanted to start a fight for refusing to buy his bracelets he was selling. We just kept walking and there was no trouble.
Have fun planning and while you're over that way you should go on a Safari in Tanzania or Kenya.