I think most of the stories stem from two incidents -- first, they they sold a batch of RAM manufactured to the wrong specification for one of the earlier Aluminum Powerbooks; and more recently they had a number of bad customer incidents, several of which were resolved by the personal intervention of the owner, involving PowerLogix processor upgrades for desktop machines. From checking out the xlr8yourmac.com site it seems that a good many of the customer complaints with the PowerLogix fiasco resulted in part from customers not throwing in the towel on the defective cards until after the return period was up -- but on the other hand customers noted that OWC was also in the position of not just being a dealer but also having an ownership tie to PowerLogix. I believe OWC has since substantially improved the return policy and warranty on PowerLogix products.
I personally have had no difficulty with them and have used them without any trouble for several years. They've taken back a CD-RW drive (for refund) and processor upgrade (for exchange) that had compatibility problems, and their other stuff has been flawless, and always very good technical support by dealer standards. The main drawback as I see it is that while their prices have gone from roughly average to some of the best in the business, this improvement has been accompanied by a worsening of their return policy -- with the result that, unless otherwise stated in the product listing, there is a 15 percent restocking fee in the event of a return for a refund (though not for an exchange).
I use them because they are conveniently close by and they have very good selection and very fast shipping -- it's much more effective than most in terms of being a one-stop-shop for Mac upgrades. The return policy change however does lead me not to use them in situations where I am engaging in a more experimental purchase.