I think they're pretty good as an extra bit of security. I use one on my front wheel and think that it would be pretty difficult to undo it without some weird tools. I wouldn't bother with one on the back as it would be a pain to align the wheel and tighten the chain so I lock back wheel and frame.
They're a lot better than the trans-x type with the five sided allen key. I don't think it's as secure as an extra lock but it's a lot less hassle and weight to carry around the place.
+1. that's all