I’ve been there! Great food! 
(I was staying in the Polo Lounge, behind the MGM Grand, and it was within walking distance. Burger heaven!)
I’ve also been to the Vegas In-'N-Out Burger. Again, great!
Reminded me of a movie line from The Big Lebowski: “That’s a good burger, Dude.” -Donny.
Yep, I went there with Big Lebowski quotes running in my head.

In-'N-Out Burger is a fine example of the Unix Philosophy: Do one thing and do it well. INO is simply a burger joint. It makes fries. It sells cups for self-serve soda (the only way to sell soda, IMHO). That’s it. Simple menu, simple concept, great food. It’s worked since 1947 and, in this age of McDonald’s and Hardee’s and Burger King, it’s all the more important to preserve good fast food. (BTW, I love INO fries. The freshness gives them a much better taste.)
Fatburger is a more Lou’s Diner-type fast food joint. (Yes, it is a ‘joint’. It deserves that appeallation. :)) What wowed me, besides the fucking great milkshakes and the magnificent burgers, was the music. They played blues! They played rhythm! They played Motown! Dining bliss! Ambiance was perfect for 10 PM on the Strip!
Nothing tastes better with real R&B than a messy hamburger and greasy fries (greasy in a very good way :)) and a chocolate shake richer than Bill Freaking Gates.
Final analysis: Don’t make me choose! Please! I wish I lived closer to the west coast, as INO and Fatburger are both completely unknown where I am.
Damn. I’m glad I live in a nation where such restaurants can exist and thrive.