C4 pits like crazy but sticks like glue. I maintained my C4 with sandpaper and paint thinner...worked like a charm. Onyx is far better IMHO, but after a season or so, it too will begin to degrade and break down a bit...certainly not as fast as the C4. I think as long as you stay up with sole maintenence and take care of them, you should get a solid couple of seasons out of the Onyx if not more.
I personally had two very bad experiences with the XS Edge that Sportiva equipts its TC PRo with, I thought the rubber sucked
, but to each his own. I sold the shoes after a pretty poor showing at Humphrey's and then again at the SOuth Butt. Someone got a good deal on a pair of very comfortable shoes. If I do it all over again, I will buy a pair of used TC pros and resole them immediately with Onyx...best of both worlds.