I don't live in an area where they have In-N-Out Burger, so it is one of those things I have to do when I travel to a city that has them.
If you're visiting and have access to a car, unless you want to visit the souvenir shop, don't go to the popular one near the airport end of the strip. This one is just around the corner near the university, and the line is NOT nearly as long as the it ALWAYS is at the other one nearby (w/ the souvenir shop).
Good burgers, melty cheese, tasty sauce. A single is more the size of a regular McDonald's cheeseburger, so I'd recommend a double cheese for the average adult.