Of course, participating in freeroll poker tournaments sounds appealing. And it actually is. You can participate for free, work on your poker skills and also win money.
The danger lies in the fact that people play in a different way than they normally would. This is because it is not “real money” so the chips they bet have no value. You should keep in mind that real money poker tournaments are different and freeroll poker tournaments don't quite show you the reality of an official high buy in poker tournament. In an official 'real money' poker tournament, everyone generally plays a lot more carefully.
Nevertheless, participating in freeroll poker tournaments can be fun and useful. It's also free; so why not?