Your mileage may vary on this one.
Purely from a math perspective, HDDR isn't a good monetary use of dining credits. The approximate "Cash value" of a single TS credit is around $41-42. So a 2 TS credit place would need to clock in around $82-84 per person to be a good monetary value. HDDR's most expensive seating is $72 per adult. I believe Dining credits are also restricted to the 8:30 PM show only.
The flip side of that is that if you DO book OOP, you have to pay up front for it. For some that's a big payment to make when still paying off the whole trip. And when you consider that it's all you can eat and drink (beer, wine and sangria included), and gratuity is included, that adds to the value if you use dining credits (tips must still be paid when dining credits are used unless the experience specifically states gratuities are included). And you get a LOT of bang for your buck with the entertainment.