mirrored forwarding virtuals: really a performance problem?

Bandwidth Controllers are a pretty new feature and unfortunately I've no experience of them. That being said, the very fact they are so new would put me off using them for a good while, personally I'd take a cautious approach and use Rate Shaping until then. I'm not aware of any potential performance or stability issues with RS but perhaps others who've used it more than I might have a different opinion. Anyone?