I have been to the Mars at Yonge and Eglinton many years ago for brunch, but this was my first time at the location - supposedly the original Mars.
This is your classic greasy spoon diner and certainly looks the part. However, it does not have the dirty / grimy feel that you may typically associate with a diner. It has bar seating along the left side where you can clearly see the cooks preparing your meal fresh to order and there are tables along the right hand side with a few booths at the back.
I arrived around 11:45, and while the restaurant was not packed, it was certainly busy and I was told to seat myself.
The menu is quite broad with all of your typical greasy spoon / diner options, and they seem quite willing to accommodate any special requests. I like that you can ask for pretty much anything on the side, for example a single pancake or a single slice of French toast - no problem!
I ordered a fresh fruit plate with a side of rye toast and my friend ordered a more typical diner meal of eggs over easy with home fries, toast, bacon, and an order of French toast on the side. The fresh fruit plate consisted of a sliced banana, a few orange slices, a chopped gala apple, and some grapes. The grapes weren't the freshest, but I was still satisfied.
Service was fairly bare because there was only one waitress serving the entire place, but she was very pleasant and was happy to answer any questions we had. They forgot to give my friend maple syrup for his French toast, and when we asked for it she apologized and brought it right away.
Overall I would return, but wouldn't necessarily make a special trip to go here.