If you go to Diablo's on a weekend or holiday, I would definitely suggest reservations. My friends and I were able to walk right in with our reso and have a prime booth in the back overlooking everyone. However there was a long list of folks waiting up to 2 hours for a table. Think ahead and get a reservation!
The Queso Fundido Dip is pretty tasty. I have the Shrimp tacos which were good, although I would probably try something else next time I went. We had a pitcher of margs that served about 8 glasses.
This place is open air and was pretty loud. If you are looking for a nice quiet environment, this is not for you.