Welcome to our forum.
Vinny and I have actually been discussing the merits of the spa blanket and we both agree that besides the energy savings from insulating, it also prevents evaporation much better than just your cover alone. The only downside is that they don't have a long life. I need to buy a new one and since mine has been off the tub for about a month, I really notice a big difference in the amount of water I am adding to my tub.So for energy savings, prevention of evaporation and extending the life of your cover...buy a spa blanket. But don't waste your money on the bubble type. Get a closed cell blanket.
How long are you all getting out of your floating blankets? We only sell the solar bubble type, but have been weighing the benefits of offering the more expensive ones as well.
IThe downside of having one for me would be when it is windy and it blows off the open cover where I have it draped. No big deal though, pick it up and put it back on!