I've had 3 opportunities to get In-N-Out this year and FINALLY was able to during my 2nd trip to Vegas...in fact it was our first stop from the airport...suitcases and all.
Before going, Valerie G. and I researched the secret menu so we can get the most of our In-N-Out experience. (http://daviswiki.org/In-N-Out_Secret_Menu)
I settled for the double double, extra toasted, extra pickles, animal style, cut in half burger with well done animal style fries. It took me awhile to get my order out and the lady was so nice and didn't rush me at all. The burger was really good...but I have to stay true to my roots and taste buds and say Shake Shack is better (although In-N-Out is cheaper). A lot of people have issues with the fries but I find that getting them well done really makes a difference. I actually like the fries better at In-N-Out than at Shake Shack.
Don't forget to ask for a free hat and stickers! =D