It's not just another fast food burger...
It's nothing out of this world, but it is a damn good burger (I think the closest thing that compares is 5 guys, only at half the price). They focus on one thing and do it well. A big reason why I love in-n-out so much is because of their business model and how they treat their employees. They pay their employees well, give them good benefits (healthcare, education, etc.), and promote from within...it's not like most places who pay some slacker teen minimum wage. That shows when you go in, the employees are happy to work there and really put forth an effort to produce a good product.
Their fries are sliced potatoes fried in natural oil, you can even watch them slice them in the store. Most people who don't like them are used to McDonalds or some other chains fries that are a mash shaped like a french fry and chemically enhanced to taste a certain way.
Compare the calorie count on in-n-out with pretty much any other fast food burger place, the ino burgers are typically much lower than any of them. I haven't stepped foot in a McD's, BK, TB for almost 8 years now...in-n-out is really the only burger I'll eat.