I'm amazed how quick my pool fluctuates in temp. It was 81 degrees, and after a 3 days of cloudy 85 day/70 night temps, the water is 77! If I plan to swim, I'll kick on the heater that morning and it'll get it up in the low 80's after a few hours.
I'm also amazed at how much heat is lost through the top of the heater (it's atmospheric). I see the heat shimmer blasting out of my equipment area. If I got a new one, I'd get a high efficiency model that doesn't loose so much heat. Something like a Jandy LXi, although I have no experience with them.