Calling Five Guys a 'downmarket' anything is silly. They're probably the most expensive / up market fast food burger place out there.
And they don't do shakes (at least here on the East Coast, where they started). Burgers, dogs, and fries (well, and some 'veggie' versions of sandwich).
In N Out is essentially the same (worse fries, but with shakes, and with customizations like animal style and the 4x4), and is religious, and has been around since 1948 (vs. 1986, and only recently in California). Five Guys charges twice as much for the same burger, though. I've tried both head to head once Five Guys opened in San Jose, and the meat, cheese, and lettuce/etc. itself was better from In N Out, the bread at both sucked, and fries at in n out were horrible by comparison (although IMO McD's fries are still the best).
Shake Shack is better than either (and, 3x the price of in n out), but is only on the East Coast.
Interesting - I had the opposite experience. I had In N Out in San Diego, and then flew to DC, and have Five Guys. I much preferred the Five Guys. In N Out tasted/felt low quality.
