I got one for my Ixus 400, and its been used more around water than under it. If not too deep, or with the help of software, the light issue can be managed, to get some resonable snaps, or you can probably use a torch (not tried yet). These were taken on my first attempt, with no additional light, at about 8-12m, without the cost and faff of a 'proper' system (sorry to all serious photographers!).
http://www.yorkshire-divers.co.uk/gallery/Tioman
I also use the case when we go kayaking, or up wet mountains, on jetboat trips, in the pool on holidays etc. It keeps the camers safe in rough conditions, if nothing else. They get my vote!