I use that too, but like many people say, it can be risky. They are allowed to take down your domain with no warning or reason, and I've actually seen that done to someone I know (paste-bin site with no offensive content). However, they do offer the service for free, so I can't really complain (recently they actual have a way of contact, but the best I usually see is resetting them resetting the domain, making it re-registrable by anyone). Personally I haven't had any problems with it yet, but then again, my sites haven't seen too much activity. It's nice for a small site like a blog, or when you're just starting out and you aren't sure of your site idea; when you feel your site will work out, just buy an actual domain since domains aren't especially expensive.
