How secure are dongles for copy-protection?

I'm looking for any information on incorporating dongles into a software package for copy protection. In particular, I'm looking for information on the Rainbow Technologies Sentinel, but advice on dongle-based copy protection in general is appreciated.

 

How easy/difficult is it to break this kind of copy-protection? Are there any known weaknesses in the dongle-type systems themselves (as opposed to implementation weaknesses?)

 

Are there any dongle-based protection schemes that have been cracked, and if so, how? (A pointer to a URL would be appreciated, if you have it.)

 

Thanks in advance.

 

HAL