Those would be ok at those prices for a card or two but their cost is going to enter the area of a larger motherboard rapidly.
These are interesting to me (though it would be better if there was a PCI-X version, maybe there is).
I have quite a few older PCs with no PCI-e slots (AGP at best), I could just buy VGAs and these adapters (and maybe in some cases a PSU) and save money (and power) by not buying a newer PC to use just for mining.
These could also be useful for other PCI-e cards...
The only problem I see is that it may not fit inside the case, since all video cards that are good at mining are full height.