I went to Five Guys for the first time (North Scottsdale) to see what all the hubbub was and why everyone was comparing it to In 'N Out burger - a place I love and have been eating at for over 25 years (CA and AZ).  

Pros: wider assortment of toppings and menu items, including the veggie sandwich (not a soy pattie - just veggies and cheese, if you want; extra menu items like jalapeños and hot sauce, grilled mushrooms, etc.; Cajun styled french fries (their small size is enormous).

Cons:  more expensive than In 'N Out. 

Neutral: Thick burgers are more like your home grilled burgers, all well done and crispy on the edges - not like In "N Out's thinner patties.

I don't know why people compare the two places since the prices are not similar.  Five Guys is good, but should be compared more to places like Fat Burger, regarding size and price of burger.

I still prefer In "N Out for taste and value.