For a few years, reviews of SAI were pitiful. Tostios chips, poor service, poor quality food, etc (note: I was not around during this time, so this is 2nd hand info). Then, I think it was early 2009 or so, they actually got told by Disney to pick it up or pack it up (paraphrasing). Since then the menu has changed, they have real chips, and service and food quality seems to have improved greatly.
So, while yes, there are lots of bad reviews out there, since they cleaned up their act it's much more balanced.
Personally, I really enjoyed it there last year. Ribeye Tacos, Sopa Azteca, and the white chololate mousse with berries were all delicious. However, it's certainly not Tex-Mex so careful perusal of the menu is highly suggested.
Funny story, as a kid I loved tacos and Mexican "stuff" (it was my favorite pavilion, and this was before I discovered the Aztec, Maya, and other pre-Colombian stuff). So, of course, I begged my parents to eat at Mexico. We eventually made the reservations at the little touch screen kiosks in WS and ate there (this was pre-1992, I think it was the trip before that or maybe 2 before it, so somewhere between 88 and 91). I can only remember there not really being anything I liked on the menu ><. (Funny thing is, we did the same for Italy (this was the 1992 trip), and I ended up with Spaghetti and Meatballs. At least, on that same trip, when we did Norway it wasn't my idea. My parents paid for the buffet (it was a buffet at the time) for me to eat my fill of red potatoes. It's all I ate ><)