> If you accept that the "actual" space is going to be less than the "advertised" space how do you gauge the risk when you buy?

It's not even something I consider, because I've generally assumed that disk capacities are relatively comparable between manufacturers. So I base it on my current space and whether or not I need more.

I don't think its unreasonable to expect that a consumer should be able to look at the specifications for 2 different products and come away with a decent idea of their capabilities. Or at the very least, not be completely misled.