I buy parts from them. Prices and selection are great. And the NAPA branded stuff is pretty good too.

I buy parts from NAPA all the time.

Generally yes. I delivered parts for them for a few years.

Yes.  I don’t get all my parts from them, but generally speaking yes.

Probably varies with the part on the house branded stuff.  Some of what they sell is the same as anyone else would carry.  I always shop around.  Advance has these 15% and 20% deals all the time that sometimes make them better to buy from than Rock Auto.  But Rock Auto is good on higher price parts, usually the discount is better even after paying the shipping.  And the refrigerator magnets are cool. 

They are not what they were years ago, but little is. You probably have a better chance of dealing with someone who knows what they're doing than at places like Auto Zone. My own preference has always been for a good locally-owned full-service parts shop but those are a thing of the past in most places. (In fact many were bought up by NAPA.)