We went last May, when the flight started for the season. Over 100 open seats each way.
It was nice, and we had clear blue skies, but it was COLD!
Temperature was about 30, but wind chills were in the teens.
We hired a car and drove the Golden Circle and then went down to the southern coast.
The waterfalls were amazing, but you could only stand to be out of the car for a few minutes.
I think it is a pretty place and very interesting. I would go back, and may do so this season.

We did not go to blue lagoon. I had always though it was a natural hot spring, but it is all man made.
My Icelandic friend said it is an overpriced tourist trap. I would still like to see it, but the natural countryside is much better.