I've used this on my Fender Telecaster deluxe downtuned to drop C which they worked great for.
The sound is really full and solid. They seem to have a high output compared to other strings I've tried, and last much longer than the standard Ernie Balls.
Personally I can't use them though, since I've recently discovered I'm allergic to cobalt. Kind of a bummer... But to anyone who isn't, I can highly recommend these strings!