Some excellent guitarists and teachers I know swear by them. They claim that some guitarists sweat acid, and go through non-coated strings every hour (such as this picker here.)

Elixirs last about a year on my acoustic; they turn a nasty shade of green about the second and third frets long before losing tone, so perhaps there is something in this acid-sweat idea after all.

I do find that after nine months or so, minute threads of the coating start decrepitating from the strings around the pick guard. Whether this fuzzy appearance gives a funny tone is very much a matter for my imagination.

In terms of economics, I find that the Elixirs last twice as long as non-coated strings, but cost twice as much, so I make my purchase decisions largely on the toss of a coin and the spur of the moment. On the whole though, I will probably stick to the Elixirs for the foreseeable future.