As implemented, they're basically useless. Asking "does X know about Y?" without verifying that I have any knowledge of Y myself doesn't do much. My dentist has endorsed me for things like "Cloud", and while he's a great guy and good at his job he can barely work his iPad.

I personally view those as noise at best and very skeptically at worst -- if someone has a bunch of endorsements for a particular skill I suspect they actually don't know much about it.