I agree w/biggedon almost totally, .mobi and .ws, avoid them like the plague, some domainers were known to lose a fortune, even in the $ XXX,XXX for one guy right here regging mobi names left and right seeing it as the future of domaining
.CC is mostly a failure, too expensive ($18-$20 in the vast majority of registrars) but it had no restrictions since practically the very start, as if it were a gTLD, and it allowed two-char domains which many extensions (most if fact) do not allow after the COM, NET, ORG era, so you were limited for 2-char domains if that was your thing and you wanted a more mainstream ext. And Chinese guys can't seem to get enough of the CC.cc names, $500+ is common for the better ones, and when was the last time you saw an LL.cc or NN.cc for auction ? or even available for sale ? those just don't go on sale, I heard that one or two entities in fact own most of the NN.cc domains, like Marchex used to do by grabbing so many of the mainstream numericals....Other examples :
.INFO does not have them (although a member here says he once saw one and it was for real, some specific CC.info regs were permitted but if the original guy sold the domain or it was moved away from its registrar then the owners of the .INFO ext would just cancel its reg for good
.BIZ didn't have 2-char's at first and then Sedo intro'd a bunch of them in a real big ad auction effort, LL.biz and NN.biz aren't bad at all, they sell real well (but stay away from CC.biz domains, those aren't special)
.US is practically a total failure too which says a lot as it's the official ccTLD for the US...I don't think there's another country on earth where its people run away so much from its own ccTLD...

out:
concerning being mainstream:
.mobi/.cc /.ws and a few other dot's didn't go mainstream either.