If you are looking for a fancy Mexican restaurant in town without the budget contraint, I bet nobody would disagree that casa loca is by far the best. They have a couple of locations in Seoul. One in Yeouido near the korea stock exchange (KRX) and the other in fashion district called Chungdam-dong or Apgujeong.
They are much more upscale dinner with real authentic flavor compared to the other Mexican places . They have variety of authentic menus that I haven't even had in the U.S. with all time favorite menus like taco and burritos in different meat types. And of course, free chips with different types of salsas for your starter. Most of my forks go there because they simply are very comfortable restarants to enjoy a night or lunch. Clean open kitchens are also comforting when you don't really know what they are doing in the back!
I believe most of foreign customers come there are business people (they look so to me), English teachers, but less foreign students mostly because you have to spend about W15,000 each meal. But like it says up there, food portions are very generous, so it's absolutely worth the money. To me, it seems even less expensive than other smaller ones.
Also I was surprised to see a Mexican chef making in-house tortillas, salsas and guacamole. yummy..... If you are looking for a place to date, family get-together, business luncheon or dinner, this place got to be the one in Seoul. One time they had huge party for Mexican embassy staffs with mariachi band. It was wild!! I hope they still have band!
If you scroll down the following link, you'll see great many articles and tv features of them along with some articles written in English. http://www.casaloca.co.kr/04-news.php
Chungdam-dong one has the valet service, in case you'd be driving. But if you are really looking for a small portion like single taco or very quick meal with limited menu choice, you should try the ones in Itaewon area.
Hope my answers your curiosity. Good luck.