I have been eating burritos at Mex-To-Go for over 10 years now, and feel like I have a pretty good handle on its place in the Silicon Valley burrito firmament.
The single best thing about Mex-To-Go is the hot sauce they provide in a large earthenware bowl. Add a little of this portable fire to any of their burritos, and be prepared for a flavor experience. I personally favor the grilled chicken burrito, finding that the heat of the chiles combines extremely well with the slight charring of the marinated chicken. My vegetarian friends assure me that the hot sauce goes equally well on their burritos, however.
There aren't a whole lot of things on the menu besides burritos. The black-bean soup is quite tasty, as are the nachos. You can get a chile relleno at times, and there are a few other dishes, but I've never really looked further than the burritos.
If you're visiting on a summer's day, take your food out back to the trellised picnic area and enjoy your little hidden corner of the Silicon Valley.