I've only tried to them once for some side steps. Ended up finding the exact same steps on another site for a better deal, and CarID wouldn't match the price, even though they claim to. They eventually let me cancel my order and refunded my payment, but the rep I talked with made it a hassle and very frustrating. Seemed like they made it easy to take my money, but hard to give it back.
There's a slim chance I'd use them again, but it depends on the item, and some definite shopping around before hand.