Unfortunately, Capital One does not report correctly to the credit bureaus and can damage your credit more than they help it. That's why myself and many, many others refuse to own a Capital One credit card.
It's a shame because everything else about them is great, I got my car finance through them.
Also, I just found this out...
FICO is pulling the plug on authorized users because there are dozens of websites that "rent" their cards out to people with poor credit for up to 90 days. Fair Isaac's public affairs manager, Craig Watts, said that beginning in September, the updated FICO scoring model no longer will consider credit card "authorized user" accounts in computing scores.
That pretty much makes Authorized Users useless now. A huge blow to people trying to repair their credit by piggybacking.