They're very nice and allow easy access to the ignition, but tend to fold into the engine cases if you drop your bike, so you may still crack your cases, particularly on the points side.
I always thought that if they had another tube with a bolt going through that tied them both together they'd be a lot stronger, but it's probably a case of aesthetics winning over practicality.
They're better in one way than the larger crash bars though (like the cops and a lot of tourers used) in that they won't bend your frame in a big "get off". Cheers, Terry.