They work great, if your motherboard doesn't do screwy things like mine. Well, maybe not screwy, just anoying. When it sees cards in certain slots, it turns off others. So I had 3 cards plugged into the x16 slots on the board, and thought, "hey! a extender will get another card in there!" But after some research, i found that the mobo disables that slot when it sees cards in the three i was using. =[
I havn't tried it with all extenders, so all the slots would be detected at x1.. maybe that would leave them all on. Not sure, threw the card in another box.

TL;DR: yeah, they work great.