They do ship slow sometimes, unless you'd like to pay extra for expedited shipping.
Almost every time, I can place online orders from DD, Buy.com, FYE.com, half.com and Amazon.com around the same time and the DD order will always be the last one to arrive.
On a better note, as for stock issues: put it in your cart and almost finalize the order. At some point it will either say "in stock, ships immediately", or "will ship in xx days/weeks", or "currently on backorder". If it's in the final pages of your order, those have always been very accurate for me in the past with them.
Overall, DD is an excellent website for customer service, price, and stock. Most CAGs place large DVD orders from them twice a year when they have their 20% off all DVDs sale, and there have never been any major complaints other than the occasional "why hasn't my order arrived yet, it's been almost 3 weeks!".