I have no experience w/ an open box mobo, but I figured I would share my opinion anyways :-). Of all compenents, I wouldn't buy an open box mobo for two reasons. First, it has a thousand different chips on it, nb, sb, sata, memory controller, etc. so if any one of those things failed it's out. Second, why would someone open a motherboard up, look at it, and send it back......if you don't need it, you're not going to open it. I find it more likely that something malfunctioned and they just sent it back without reporting it. Use your own judgement.
The open box MB is not guaranteed to to have all the the componets and assessories that would be in an original factory sealed box. The MB might be ok but missing some of the goodies. I wouldn't get one.
You coulda specified that in your first post and saved the trouble... Why not go for it, they have a return policy for refund only so at least you'd get your money back if it didn't work. But like others have said, it's the motherboard... the most integral part of your computer. I could see getting an open box OEM dvd drive or something but not a motherboard.
I bought a newegg open box long ago for socketA, and it worked as new but did not come with the i/o shield but I eventually found one that fit. It was a $19 pc chips m811lu and it has been one of the fastest athlonxp mobo's i have used, of about 12.