I hated Rex as much as the next guy here, but Karros is definitely not the answer to any of the problems in that department. Other than simply having a less annoying speaking voice than Rex, it's probably worse in every other way.
It has a very tacked on sound to it, and hardly ever sounds natural. Plus, it doesn't help that Karros sounds completely different than the other two guys. I don't know if they recorded him in a different manner, but whatever it is, awkward sounding commentary is the result.
On the plus side, this is basically the only gripe I have so far... everything else is great.