I agree with a lot of others. The food is not the best Mexican you will ever eat, but in my opinion (as well as DH's)...the atmosphere in the restaurant makes up for it. The dim lighting, the boats passing by, the band playing in the background...it's just a really nice place to sit and relax and feel like you are actually in Mexico.
Last year, my DH had his first trip to Disney and loved it there so much that when making our plans for our 09 trip, I asked if there was anywhere from last trip that we HAD to have an ADR for (if he could only pick one)...he picked San Angel Inn over Flying Fish Cafe, Les Chefs de France, Hollywood & Vine, ESPN Club (SHOCKING!).
Good luck - I'm sure you will not be disappointed.