I've had problems with a few sellers, but those were usually resolved in a timely manner. My biggest problem is when I have a problem with the site itself. Their customer service isn't all that good. And, with Paypal as the default setting for payment method, it's gotten pretty hard to find sellers who accept anything else - I have had bad experiences with Paypal and can't bring myself to trust them anymore.
When I can find a seller who accepts other payment methods, though, I'm perfectly happy with doing my business on ebay.