First of name zero was a big scam in the internet history... In fact number of people booked the domain through them, finally no one has the ownership.
Same applies for dot.tk. Dot.tk is just a referral or URL cloaking, you do not own the domain. You do not have the rights to change the domain settings and so on. You can only point it to some web page or some other site.
In case you want to purchase the .tk domain its not a very attractive option. Internet is still driven by top level domain like .com, .org and .net. .us has made some good impact as most of the people are going for that.
Overall its ownership of domain and presence on the net. I will not suggest dot.tk, as you do not know till when services will last.
If I have to vote, I will vote for uni.cc. We can have review on uni.cc at some later stage.