Just ate at Isabel's in Lowell the other day, it was excellent. Very fresh and huge portions. It was lunchtime on Friday and there were about ten people ahead of me in line and we still got our food in about 5 minutes. I had the mexican plate w/carnitas, my girlfriend had the soft tacos, we both loved them. I have been to the Mambo Grill in Downtown Lowell and thought it was ok but a little pretentious. Everything was a couple of bucks more than Isabel's, and we also waited a half hour for one burrito and chips and salsa. It seems to draw kind of a yuppie crowd so I would head to Isabel's if you are serious about food. It is right off the Lowell Connector in the Hannaford Plaza.