How have I not written a review of this place until now? I live just up the road and I've been here several times. Big M has been around since 1965, serving up good no-nonsense burgers, steak sandwiches, souvlaki, shakes, and many other favourites.
For me, the thing that makes their burgers special is, they don't try to make into something else. It's just a burger, a good beef patty and your choice of toppings. It's the KISS principle - Keep It Simple, Stupid.
These burgers are better than any of the "higher end" (more expensive) burgers within miles. They've been doing it right for over 50 years! No need to change anything.