In my opinion, this is one of the better, fresh-and-healthy Mexican restaurants in Pittsburgh. I have been going to Fajita Grill for about four years now, and I think that the service has improved exponentially. It really used to be terrible, but it seems that they have addressed the problem on my most recent two visits. I still probably would warn against going here on a tight schedule though. The food is as delicious at it ever was.
It can be a bit pricey, but the restaurant is BYOB, but with a steep $5.00 per bottle corkage fee. There are two liquor stores within walking distance. There are also several bars where you could get six-packs of beer, so I don't know why the BYOB policy (a standard in liquor-license stingy Pittsburgh) is mentioned as a problem for some reviewers.
I recommend the fajitas with shrimp and grilled vegetables, the enchiladas with Cancun chicken and mole sauce, and the grilled cheese and chorizo appetizer. Also, the outside patio is really nice.