Regularly since the 70s. You need the right clothing, one day can contain many weather variations, the furniture time I went, in July, I had to go out and buy warm clothing.
The island is beautiful, but keep away from Douglas, I find it run down and depressing. The fishing ports in the south are lovely, Port St.Mary and Port Erin, which has a steam train in season running to Douglas. From Douglas you can take a trip on an open tram car to the north, it has spectacular scenery.