No problem. I've not heard of anyone having a problem. Some of the drives do have a jumper to "Force" the drive to operate at SATA II, but even this is rarely required.
6 Gig HDDs is more a marketing gimmick - Hay I've got the lastest greatest interface! But the only performance increase is in the "Measured" burst rate. After the "Burst" has been completed, a Mechanical HDD does not even hit SATA II capabilities.