It turns out Northgate is home not only to El Mercadito, but to three--count 'em, three--taco trucks (east side of Northgate between West El Camino and Haggin): La Mex, El Neco, and La Piedad. I and my assistants (my 13 year old son, and my parents) tried the first two. We split 4 tacos from each truck: asado, pollo, and adobada from both; carnitas from La Mex; and chorizo from El Neco.
All of the tacos are very basic. La Mex serves them with onion & cilantro in the taco; El Neco serves them with just the meat, and has onion & cilantro available on the side. Both trucks have salsa bars. Tacos are $1.25 at La Mex, and $1.00 at El Neco.
The consensus was that both were very good, but we liked La Mex a little better. The meat was a little more flavorful, and there seemed to be more of it. My favorites overall were the adobada (pork, marinated in a spicy sauce and roasted) and the chorizo (which La Mex doesn't offer). I wasn't in love with any of the salsas (hot red from both, mild green from La Mex)--the basic canned Herdez Salsa Ranchera is just as good, IMO.