A group of friends and I dove at Silfra with Dive.Is on May 17th - what an amazing experience!
To begin with, Dive.Is is an osom outfit - very well run, with a great group of divemasters who are fun and helpful the whole way through, from booking the rez to arranging transport, as well as, of course, on the dives. If you dive in Iceland, at Silfra or other sites, be sure to use Dive.Is - ask for Hana, Siobhan, Jennie, Reynar and Pascal. Beware - they have terrible senses of humor.
The dive itself is a wonder - the group I dove with is very experienced, including 2 marine biologists, and we all talked about how it was unlike any other dive we've ever been on. The dive site is in a glacier lake *in* the continental divide between the North American and Eurasian plates. Yes, the water is cold! But the dry suits keep you warm enough. The profile itself is stepped and varied, and the viz ranges towards 300 feet. At some spots you can touch both plates with your hands as you navigate the route. In addition, the surrounding countryside of the Thingvellir national park is beautiful and mountain-ringed.
All told, if you scuba dive or snorkle, or are just straight-up adventurous, I highly recommend setting up a dive at Silfra if/when you visit Iceland.