This is the best Roberto's in Vegas. I have tried many and I think the one by campus is the most generous with cheese, guacamole, sour cream, etc. It's a lil hole-in-the-wall but it's open 24 hours and has great, cheap food... relatively fast. Can't beat that!
When I lived in Vegas, I used to go here at least once a week. Sometimes more!
Try the Horchata too. It's a Mexican rice drink (with a lil cinammon). It's yummy and good for the stomach if you've been drinking!