I insulated my brewpot to get a better boil, but my 20' length of 3/8" copper immersion chiller was no longer good enough without going through an immense amount of water and time.
I took the existing coil, and attached two more coils of the same length in series for a total of 60' x 3/8" copper immersion coil.... it works really well.
It was not cheaper than a plate chiller, and actually more costly than a DIY CFC, but I decided on the immersion route because of: ease of sanitizaton, no possible clogs in chiller.
I chilled from 212F to 70F in short order (not sure exactly how long, but it was quick)