Borrowing someone's car—am I covered enough?

I borrowed my friend's car last weekend (mine was in the shop, long story), and it got me thinking about insurance. I mean, I know my friend has insurance, but I started wondering if that's enough if something actually happened. I ended up looking into this extra coverage thing for drivers who don't own the car they're driving regularly. Seems like a good idea, but I'm not totally sure if it's worth it. Anyone else looked into this or have experience with it?