Rock is a good company. They're primarily a drop shipper, listing real-time inventory of various large wholesale warehouse across the country. You place your order, it ships out of that wholesale warehouse, or direct from the manufacturer's distribution center.
The big stickler for me is the multiple shipping charges if you want to order multiple parts that are all in different locations. This is the trouble with a drop-shipping company, they're decentralized, and with the low profit margins they can't absorb the additional shipping cost so they pass it directly to the end customer.
FYI, Amazon uses many of the same drop shipping warehouses that Rock uses. The upside to using Amazon, even if the price is a little higher, is the free shipping over $25 (make sure it is a part actually sold by Amazon itself) and no additional shipping fees if the parts come out of different warehouses. Thus I tend to use Amazon for very large parts orders or orders that are very heavy.
Rock gets my business for lighter parts that they show all shipping out of the same location.
Although, lately, I will admit that Advance Auto's online promotions have made them very competitive. Just bought a motor mount from Advance that was $44.95, minus $15 off $40, brought the total to $32.35 with tax... zero wait, they had it in stock at my local store. Rock has the same brand mount $23.79 + 7.53 ship = $31.32... would have saved a buck, but woud likely have had to wait 2-3 days for the mount.
Even though Rock's pricing is really good, factor in shipping and B&M store discounts, and it definitely pays to shop around depending on the part and the urgency of repair.