I've been to Burgatory with large groups on several occasions and always had a good experience. Making your own burger is pretty fun and the veggie burger is great! The mixed drinks are strong, so beware if you are a lightweight like me!
The milkshakes are delicious (and you can get "adult" versions that are spiked), but I never have room for one after I'm done eating. I've found that getting 1 or 2 milkshakes for a table of 4-6 tends to be more than enough for everyone.
The restaurant itself has a great design- LOVE the design of the menus and the wall graphics, etc. Friendly and relatively fast service - no complaints here! Lots of parking, too.