On the other end, everyone does know .tk ... I think it's got its value only if you have a good keyword. It will be easily memorisable, unlike other 'tropical' extentions. Of course the ads and such are a minus, but on the other if you see how many people visit .tk sites ... the reputation won't stop you from getting a good number of hits.
But only valuable when having a good keyword.
PS: in the Netherlands some people tried promoting it as an abbreviation of Te Koop, which means 'for sale' ... never really worked though.