I applied online for an auto loan with Capital One Auto Finance (capitalone.com/autoloans) and got approved for a low rate. They sent me a Blank Check that I can just sign over to the car dealer and cut out the usual auto financing hassles. It lets me negotiate like a cash buyer and ensure a low auto loan rate. It almost seems to good to be true. Has anyone else had favorable or unfavorable experiences with Capital One online auto loans? Thanks!

It is legitimate. What they do is once you choose your car the auto dealer calls Capital One and it goes from there. You still have not cut out many steps and may end up with a payment that you are not satisfied with or a higher interest rate. Read the fine print.
A better thing to do is find the car and find out what the dealer can offer, then walk away and go the Credit Union (get a membership, if you do not already belong). Take the info on the car and shop around for the best finance deal.
Financed cars is more expensive to insurance cause it requires full coverage. You can compare how much you would pay for full coverage of this car using this tool – carquotes.sinfree.net
Capitol One is one of my largest lenders. They are also one of the largest lenders in the United States and report to all of the credit bureaus.
A comparison to whats available is good too..
http://www.surveyland.org/jump.php?link=auto
All the best to you.