You should definitely avoid them if you have clean Bitcoins. There are two reasons for that. First is they are very expensive. You will get much less out than you put in. Second is you might end up with Bitcoins someone pay to hire a murder with.
It wouldn't be a very good mixer if you could tell the origin of the coins you ended up with.
Most coins that go through mixers are clean, but they have a lengthy trail that would identify everything you've ever done in the past. That's the main point of using one.