All co.cc domains were delisted by google on mass recently, due to massive abuse by spammers and other web trash. Similar free domains that have little barrier to misuse will likely meet the same fate, so they're not worthwhile for any serious use.
I agree with oknedar, Google has taken to blacklisting many of the free domains, for obvious reasons. My question is this: if most hosting now comes with a free domain name, is this even an issue? Hosting usually starts at like $2 to $3 a month... Might be worth looking in to.
I used a free domain and hosting service as a testing ground for my site before I launched it properly with a TLD and paid hosting. It was useful to play around with different things on a live site, confident in the knowledge that no-one else in the world would probably see it. But other than for that purpose I don't think a free domain is worthwhile having.
No Instead Of This You MAy use Co.cc Or You MAy Use blogspot Its Better...
cu.cc is out there and apparently it is free ( I haven't tried it but it says so on their page http://www.registry.cu.cc/ Yes there is blogspot, there is also yolasite, weebly etc etc but the question was for cu.cc. Whether it is "ok" depends on what you mean by that. If you mean will you ever get any visitors or ranking then the answer is no.