I went down to Havelock for a few days in Sept 2013 - was with the family, so I only got in a couple days of diving. Some of the shallow reefs had been bleached out, but the slightly deeper spots were still healthy. First day the conditions were great - second day, there was crazy current, so only did 1 dive. We stayed at an (somewhat expensive) eco-resort on the less-populated side of Havelock, which was really nice (and next to a rather deserted beach). The side with the dive shops was much more built up. I dove with Barefoot - the rental gear was fine, and the DMs were great. Though you had to trudge through the water a bit to get to the boats, so it's a little more "roughing it" than some resorts.
Look out for the weather - there's a definite monsoon seasons. We were on the shoulder, and had a couple days of rain.
One thing that disappointed me is that it seemed impossible to do 2x2-tank dives per day. All the shops I spoke to only did a single 2-tank dive (and possibly a 1-tank night dive). So it's not the place to go to if you're looking for a all-diving-all-the-time trip.
---------- Post added February 7th, 2015 at 11:11 PM ----------
Oh, and because of travel logistics (flying in and out of Port Blair, boating over to Havelock, and the associated flight/boat schedules), you end up losing a bit of time. I think you end up having to spend one night in Port Blair on the way back - seems like the island officials purposely schedule things for that, to "force" you to spend time (and money) there.