investment wise? no, that extension is worthless.. there is about 250 country code suffixes last time i checked and there is absolutely nothing special about .tk -- if you get a generic domain in that extension the whole point would be to develop and establish it... not to hold onto it as an "investment"
I would say .tk is worthless. Most sites that have them are kiddie sites hosted on geocities or freewebs, and you would likely get very low type-ins even with a premium .tk such as dating.tk.
i regsitered a three free .tk and forwarded them to a developed long name site and people remembered them as they are short and generic and continously type them
although they are getting good traffic ( more than 20 uniques daily )
i don't even think about paying for them at the moment
i would say if you are a pure domainer and not developing sites forget about them
but if you have a developed site with quite long name and you find the relevant generic name available at .tk why not get it as it is cheaper than other country tlds only 6.59 $ if you registered the name for 10 years
I disagree that .TK extension is worthless. It may appear worthless because of its FREE .tk. But those free version is actually the channel of getting a value on the paid .tk.
I dont think webhost.tk, domains.tk, internet.tk, sex.tk are worthless.