I currently have one, and was wondering how people look at them in around the world. Needless to say they are free, and have never been down (at least not when I go to it).
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If it ain't broke, fix it until it is!
tk are popular as free domain names but you can always opt to get rid of pop or frame publicity, and eventually pay for it to gain access to DNS settings and other domain features.
This is much likely WS domains, it's all about what you plan to do with it.