I'd say go for it. My friends have done it and it wasn't bad at all. I think one of them had the****s for a few days, but that's it. Easy money for a fairly small risk. I'd go in and ask what kind of level its at though and see what kind of testing its been through first though. If it sounds dodgy or you don't like it, then don't go on.
Or you could perhaps participate in some medical surveys. I was paid £25 today for giving up 20 mins of my time to read a new drug leaflet and answer some questions about information it contained (new law basically states that leaflets included with drugs have to be tested first...). This is the safer alternative, but with less money obviously.