Yeah--burgers aren't steak, but if they are using thick patties and fancy beef, they have to be done right.
If it came out super undercooked, I'd be tempted to send it back. I like color in my burger, but I have had a couple of burgers recently that came out what I would just call "rare". It can be hard to judge since one place's medium might be another place's medium-rare...but nobody is going to fault you for not wanting to eat raw meat if you didn't order it that way. I haven't sent one back yet...just remembered to order it one doneness level further if I go back.
On the overcooked side...I dunno. It would take more for me to return it. On one hand, if it was a good place with quality beef and toppings, it should still be good if a bit overdone. Kind of harsh to return something that still tastes pretty good for being a few degrees too hot.
On the other hand if you went to a place known for their fancy burgers specifically to cross that burger off your list, and they came out with a burnt chunk of beef? That sucks and is going to ruin your meal.
I'd probably cut it in half and take a bite from the center...if its really super dry and just not good (and cost $15 or more), I'd probably be willing to send it back. And at that point, you've got the other half, sliced cleanly through the center, showing an obviously overcooked burger.