I tried Seven Lives a couple of years ago after someone told me "it's the best baja fish taco they've ever had". Wanted to test that claim. Fast-forward two years - I've tried many different taco places around the city (La Carnita, Wilbur, Tacos el Asador, etc). Seven Lives remains my favorite. I was also in San Diego recently (the home of the fish taco!) and had a chance to try a few taco places there. Seven Lives is definitely comparable.
I would stick with the classic Baja Fish taco if it's your first time, but do make sure to try the rest of the menu. Their salsa bar has only three options, but all are very tasty. Beware of the hot one - it's hotter than you think :) Prices aren't the cheapest, but worth it for what you get. 3 tacos should be filling for the biggest appetite. 2 is a good amount for a light lunch or dinner.
It's a pretty small restaurant (one table for seating). Most people stand or get take-out.