I have been working on several fairly major projects over the years, a 1997 Jeep and a 1998 Ranger, as well as a 1991 Honda CRX.
I have bought a large amount of parts from Rock Auto, about $1000 plus. I used to use Advance Auto parts, but got fed up with the rude and combative staff. I also carpool so its a b**** to get to a parts store unless I make a special trip.
Some impressions:
The website is usually straightfoward as to the part you are getting. Usually. I ordered some front springs for my Ranger, so I ordered two springs. I got two boxes of two springs... . I ended up spending, with shipping, MORE on clutch parts for my Ranger than I would have from a parts store. The parts store had a complete kit which included everything I needed. I found this out when I needed to chase down parts when we broke some parts we intended to re-use. I spent $150 I didn't have to in clutch parts and shipping, which got frustrating... Then I went to RockAuto for the part I broke, (master cylinder and resivoir for the hydraulic clutch) I ordered the part with expensive rush delivery for around $100. They didn't process my order till two days later so I didn't get my part till Monday instead of being able to fix it that weekend, I was very unhappy. In addition, I ended up ordering extra bushings for the leaf springs, because I couldn't tell there were bushings with the shackle kit.
When I bought Jeep parts, in contrast, I had a fairly good experience. But it was all really simple parts, not assemblies with vague diagrams.
Its difficult to ascertain the quality of the part you are getting. If you get it at a parts store, generally you can look at it. With Rock Auto, you have to know what you are buying. If you see the part, you can sometimes make the call to upgrade to a premium part. The descriptions of the parts are limited and you have to research carefully to make sure you are getting the right part. This can be extremely time consuming, whereas at a parts store, they bring you the dang thing and you can look at it.
Another frustrating thing is, stuff ships from different warehouses. So you might order two stablizer bar arms, and they will come from different warehouses, and one is shipped by fed ex for eighteen freakin bucks. Why? We may never know.
Its nice to get stuff to your door. They include sweet refrigerator magnets, things are packed well, and I've never had anything broken.
In general:
Unless its a big ticket item that you can get at a significant discount, or a big job with multiple related parts that ships from one warehouse, I hesitate to reccomend Rock Auto. With shipping, the savings is insignificant and as I am finding more and more, MORE expensive than the parts store unless you order from all one of their warehouses. I had a couple good experiences but the last few have been sour. Being in a hurry for your parts is not a good idea, either. They ship from all over hell. Some will come the next day, others not for a week.