We left it until the day of the tour to book this, as the weather forecast had been pretty poor. The gods smiled on us though and the weather was perfect. We had a few problems finding the place - the old harbour is quite large and there are lots of whale watching companies dotted around. There's a big map of companies' locations on the harbour itself, but Reykjavik Sailors isn't on it. I called the office number advertised on the website, and the helpful receptionist gave me excellent directions from my position.
The boat was crewed by an all-female crew who were friendly and knowledgeable. Of the three, one piloted the boat, one sat above the bridge looking for marine wildlife, and the third wandered the deck pointing out facts to the guests. The boat itself was relatively small, modern and fast - we overtook quite a few of the other vessels leaving the harbour. I think there were probably only around 20 guests, so it was quite a personal experience.
We were extremely lucky. We had a reasonably close-up visit from 3 minke whales. The real experience was the white-beaked dolphins though. We were visited by 3 different pods, and one of them stayed with us for quite a long time, breaching and leaping in the bow-wave.
In short, thoroughly recommended - one of the highlights of the trip.