Is there any catch with free domain services?

[hiigara]

I signed up for a free .tk domain.
I works fine, no popus or anything. I can even use my own nameservers.
What will happen now? Will they start charging me when they feel like?

It seems I better use it only for temporary websites or I might get screwed like this guy:
http://dottkscam.blogspot.com/2010/02/dot-tk-giant-sedo-parking-scam-stealing.html

[Chris]

1) They are honest but provide it for free and will go bankrupt, your domain goes down with them
2) They live of adverts on the "get a free domain" page, which means they should shut down domain that are used and consume their resources
3) Even if everything is all right, what you intend to do with .tk, while everyone intuitively google for .com when looking for the website?

[Marek]

It costs about $10/year to register a domain, which is almost nothing. It's definitely worth getting a real domain.