The best I've done on Carbon is 8th in one of the Omaha Hi freerolls, though I regularly make pretty deep runs. I think the key is, like with any freeroll, play tight in the beginning, and a little looser in the later stages of the tourney. Compared to other freerolls that I've played in (FTP's comes to mind), Carbon has a pretty hardcore blind structure, so you really need to make the cards work for you as best as possible.
Anyone else notice how the freeroll schedule has changed, though? I miss the Badugi freeroll (and late registration didn't hurt, either...). I'm also curious about how the payouts work, now, since it looks like that changed, too. Has anyone actually cashed in one of the Carbon freerolls lately?