I went the cheap route last spring and bought a 4x20 mat off of Ebay. I have a nice southern exposure so it performs well for my 15x25 oval. Most of the summer here in WI, the pool was between 85-88. It sort of spoils the kids as they then think 80 is "cold". We experienced a warm (for us) summer last year so the pool was used almost every day. Many times, I didn't even cover it at night as the water would warm right up by noon.

Install was easy - I made a rack out of plywood and 2x4s. Diverter valve enables me to shut off flow to heater when the water gets too warm (it happend a few times last summer). 2 of these mats would be needed for a 27' round.