Stick with the Margaritas
Margaritas are great but inconsistent. One night 4 of us were there and ordered the same Margarita. however all 4 tasted and looked different. The food is mediocre at best. Not especially flavorful. One night I ordered fajitas and they gave me quesadillas instead. They were filled with what looked like cooked frozen vegies (cauliflour, shredded carrots and broccoli), and the cheese was boring. I will not go back again.
Food: 2
Service: 4
Ambiance: 6
Overall: 4

3 Years Later, Steve Is Right
Lousy service - the entire place from the bar to the kitchen has a mañana attitude.
Food: 4
Service: 2
Ambiance: 6
Overall: 4

Awesome
Sierra's is hands down the best place in MetroWest for Mexican, and the best Margarita's as well. Their standard margaritas or a custom one you tell them to make - awesome.
Food: 10
Service: 6
Ambiance: 8
Overall: 8

We Love Sierra's
My boyfriend and I go regularly to this small, intimate, romantic restaurant. The dim lights, great drinks and festive food make for a great date place. It's a little warm inside, a little noisy, a little crowded - just what we look for in a great Mexican restaurant. You gotta go!
Recommended: chili
Food: 10
Service: 10
Ambiance: 6
Overall: 10

Great Drinks
We order take out on a regular basis. Susannah is very nice. My husband and I love having a drink while we wait for our food. Could not ask for a better ending for a hectic work week. Good drinks, good company and good food.
Recommended: Tacos Sierra
Food: 6
Service: 8
Ambiance: 6
Overall: 6

It's Good
I'll echo what I have read before. Small, noisy, good food, excellent Margaritas, and small parking lot. The service is just OK. Good place. If you like Mexican/Southwestern it's well worth the ride.
Recommended: fajitas, chimis
Food: 8
Service: 6
Ambiance: 6
Overall: 6

Sierra's Is Good
I agree that the first guy, Steve, is wrong. My wife and I lived in the southwest for nine years. Now Sierra's is not as awesome as a couple of our favorites out there, but it is by far the closest thing we've found in all of New England so far. The 'ritas were good and all of the food was good, and had much better authenticity and originality than other cheesy chain/family style Mexican places around here. They could improve the outside appearance a bit - you'd drive by it if you weren't paying close attention. Also, it is tiny, but is almost better in that way, making it somewhat of a hidden gem.
Recommended: enchiladas
Food: 8
Service: 8
Ambiance: 6
Overall: 8

Sierras is great! I don't know what that guy [Steve] is talking about. Margaritas are excellent and potent. I always get the enchiladas which always are great. Mix it up with 2 beef and 1 chicken for ultimate pleasure. The place is tiny but that is what is cool about it. Sometimes you do have to wait in a real small bar area, but take that time to get your fade on with a margarita or 2. The only thing that is sometimes sub-par is the service, just not enough attention. And when ordering a drink from the bar be patient, cause that large man behind the bar is in no rush. Overall the place is excellent though.
Recommended: enchiladas
Food: 8
Service: 6
Ambiance: 8
Overall: 8

We Liked It
Yes, it's somewhat small, crowded, and noisy - perfect setting for top notch margaritas. Crab cakes as an appetizer were pretty good, the Mixed Texas Grill (Chorizo, chicken, and beef all in a tangy bbq) was great, and included hominy in addition to the typical staples of beans and rice. My wife enjoyed her chicken and mole sauce - and not many restaurants do a good mole sauce. Try it out if you don't like mega chain Mexican.
Food: 8
Service: 8
Ambiance: 6
Overall: 6

Don't Bother
Blah... When will Metrowest get an above average restaurant? Margaritas were good. Tortillas were cold and salsa out of a jar. Calamari appetizer was good but the fajita was average. Seafood burrito had chunks of partially frozen canned lobster meat in it. Waiter never bothered to come by after meal was served. Crowded with people and poor parking.
Recommended: drinks
Food: 4
Service: 4
Ambiance: 4
Overall: 2