5 guys do smash their burger! I used to eat a lot and free! We used exchange free coffee for free meals with them in my old job! I've been once in Meat Liquor (that one in covent garden) and was very good! Wendy's, Hardee's, In 'n' Out... it's a shame we don't have here! Apart of 5 guys (people used to say the british one is way better than the american one) I think all these american chains lose a bit of identity (taste, price, size of portions ...) when move to UK! I tried Taco Bell here and was disappointing! I had in America and I remember that was cheap and massive portions!5 guys is good and they only have meat in the burger (which is how it should be). If I remember correctly they also smash the burger but do it after they flip which seems weird to me.
I think they are over processed, the meat flavour doesn't really come through because of this. The best burgers I have tried are fresh minced beef and salt/pepper added just before cooking.