I had heard so much about the Blue Lagoon that I felt I HAD to go there during a short trip to Iceland. I was very disappointed by the experience.
At £100+ per couple I had expected something very special, unfortunately I experienced a badly run tourist trap. We had had a long day and wanted to eat before relaxing in the water but at the crowded entrance we were told that we had to go through the changing rooms first and dinning facilities were available "inside". Wrong. At the end of the day we found a lot of people had in fact visited the restaurant first.
Prospective visitors should be aware it cost £20 to hire a towel and bathrobe (to be fair, a "free" drink is also thrown in). What a rip off.
You should also be aware the water is not blue. The high mineral content means that water is milky white and it is impossible to see your hand a few inches below the water surface.
Do yourself (and your wallet) a favour and visit one of the many hot spring spas within Reykjavik. I am certain you will get a lot more out of the experience.