while experiences may vary from store to store i can say that i've always had good experiences dealing with them.
as is typical with retail, not everything may be the lowest price that you will find compared with online. they do pricematch online retailers but only for sale price + shipping total cost which is fair. this does not include after mail-in-rebate prices as they cannot match those as its a special deal given to vendors from the manufacturer. their prices on cpus are generally the lowest around and they have fair deals on bundles. for everything else prices are generally average although there are always some nice deals to be had.
you can likely save more by buying some parts online and some parts locally however its really convenient to buy everything from one store.
just like other retail, there is no discount for buying everything from them other than any advertised deals or bundles.
stock of parts is generally good, although this depends on the store, demand and general availability. i've heard of a few stores where it was an issue but i've never seen it at mine.
i've never had to make a return, although my store's customer service has always been fine.
as for knowledgeable employees, it depends on the person. some really know their stuff while others not so much. the place does attract the geeky type though so more often then not they at least have half a clue.