I think a large part of your perspective on the blue lagoon will depend on what package you purchased. We purchased the "premium" (mid level) package that included towels, slippers, robes, masks, and a couple of free drinks. We arrived for our reservation and there was no line for premium at all. We started with lunch in lava and enjoyed our free sparkling wine included as part of the package along with our food - the food was very good (yes, everything is expensive but it's on par with costs in Reykjavik restaurants).
We then went to the locker rooms to change and get into the pools. As some of the other reviews have mentioned the locker rooms were really quite chaotic. We didn't have that much trouble finding lockers though and once you figured out the locking system it was convenient to have it tied to your bracelet.
We spent several hours in the lagoon and ordered several drinks from the in pool bar. The scenery is beautiful. We also did our mud and algae masks which were nice but didn't really do that much for our skin. We very much enjoyed our time in the lagoon. In terms of temperature the pool is warm throughout but some sections near where they pump water in get much hotter, so you can move around the pool to find your ideal temperature. I did not find the water was so hot I felt like I had to get out frequently as I usually do with a hot tub.
I would avoid getting your hair wet with the lagoon water if possible and if you do make sure you wash it thoroughly. We left the conditioner from the showers in our hair as recommended and did not have lingering hair issues after shampooing and conditioning after leaving the pool.
All in all we really enjoyed our time here and would do it again. We spent about 1.5 hours at lunch and another 2.5 in the pool, with locker time we were at the blue lagoon for over 5 hours.