I always try to hit this place every time I'm in Vegas. Owned by Mary Sue and Susan (I know of them from their radio food show once on KFI Los Angeles), this is a GREAT Mexican food restaurant, from ambiance to the sauces. Especially the salsas and sauces.
Tip: If you're only at Mandalay Bay to visit them for lunch or dinner, park in the Convention parking lot, as they're close to that lot, near the Shark Reef. Otherwise, you'll have to trek a ways from the main casino (which isn't bad, considering you can walk off the calories on the way back).
Tip 2: If you like really really hot salsa, like the addictive kind that makes you cry but you just can't stop eating it, ask your server for the "diablo" salsa.
Tip 3: If the climate is decent (sunny, 70's-80's) ask if you can be seated out on the patio for nice ambiance.