NY,
There have been a few posts on this subject. I also figured that it may help in the insulation and cover wear departments. Haven't gotten our second electricity bill yet, so I can't comment on the cost differential. We live in So Cal, so my guess is the savings would probably be minimal.
I spent about $25 at a Leslie's for mine, and then custom cut it to fit. I spent quite a while with a pair of scissors, carefully trimming it to the contours of the tub a little bit at a time. When it was done, it fits almost perfect. I'll take a picture of it this afternoon and post it later today.
As for the removal, it is a little bit of a pain. My current method is to open the first half of the cover, then pull half of the blanket up on to the top. Then I simply use the cover lifter to pull the rest of it off the water. If it is still hanging in the water, I just tuck it outside the shell edge, between the cover and the shell. Putting it back in is simple: Drop the cover lifter down, and the blanket just slides back into the water.