If my wife and I can claim any single restaurant as "our place," this is it. Like a number of restaurants we discovered when we were dating, we first went to Los Güeros having found a coupon for them in the Entertainment book, but we really got attached to Los Güeros during our premarital counseling in Arcadia, after which we would go just a little further down the street to enjoy dinner at Los Güeros each week. A year or so later, when we moved to Monrovia, we were thrilled to discover that our new apartment was just around the corner from our favorite restaurant!
It's true enough that Mexican food places are fairly plentiful in Southern California, of varying degrees of quality. Besides the fact that Los Güeros simply provides "good food," one thing that makes them really stand out is the guacamole cart. The cart will come right up to your table, and you can tell the server exactly what ingredients you want to include in your guacamole (we add everything except the peppers, since my wife doesn't have a very high tolerance for spicy foods), and you can watch the guacamole prepared right there in front of you. Guacamole doesn't come any fresher than that!
The prices are quite reasonable, but this is further enhanced if you bring a coupon, which generally isn't at all hard to do. Besides the one you can get from the Entertainment book, similar "free entree" coupons are easily found in local publications, and the servers will occasionally provide some at the restaurant itself as an incentive to return (not that you're likely to need much arm-twisting!). If you're going to bring a group, I recommend the coupons you can get at Restaurant.com, but the dinner coupon requires a minimum purchase of $40, so it's not really viable if there are only two of you (but if you reach that minimum, it's totally worth the effort!).
We usually go to the original Los Güeros location on Huntington (that's the Route 66 one), but the restauant has been so successful that they've opened up a second, larger, location on Myrtle in Old Town Monrovia. Either site is worth checking out.