I would just say manage your expectations. In n Out has such a devoted following and giant hype behind it, that it might be difficult for the burger to live up to those expectations. That said, I think it’s an awesome, awesome hamburger. I remember liking Fatburger slightly more, but I just keep coming back to In n Out when I’m in the area. I’m addicted.
But, remember, the main thing with In n Out is that it’s a very basic burger cooked on a flat top grill/griddle. The beauty is in the perfect simplicity. The meat is good quality and freshly formed into patties (never frozen). It’s griddled to a perfect, slightly crispy and caramelized edge. These are small patties (either 1/6 pound or 3 oz, I think), so they generally cook up to well doneness, which I think is appropriate for this style. Cooking it to a lesser doneness would not allow for that outside caramelization to occur fully–at least in my experience it would be difficult. That said, looking on the web, it appears that it is possible to request the burger at a lower level of doneness.
The buns they use are just the right fluffiness and lightness to complement the burger and, if I’m remembering correctly, they’re briefly griddled, too, so they get a slightly crispy edge, as well. I think a double is the right ratio of bread to meat. The rest of the ingredients are just the typical onions, lettuce, tomato, and “spread,” which is a Thousand-Island-like special sauce. All taste as fresh as the burger.
So, don’t go in there expecting a giganto half-pound patty, grilled over an open fire, topped with Roquefort, truffles, and cracked black pepper aioli. Just think simple, basic California/diner-style burger, and think of In N Out as quintessence of this style of burger.
As for the fries, they’re solid. Very fresh-tasting and potatoey. I like my fries to have a bit of skin on them, but that’s a personal preference. Like the burger, the fries are cleanly and impeccably executed.
Oh, and the service. In n Out may be the only fast food chain I’ve been to that, no matter which outlet I go to, all the employees act like they actually want to be there. I mean, they look positively happy to be working there.