I've been here twice, once for lunch and once for dinner. For lunch I had a BLT and for dinner I had a butter lettuce salad and some kind of small plate with chili sauce and a poached egg. The BLT was delicious while the butter lettuce was really sour and the egg dish was really salty. Based on that limited information, you can probably conclude that the Mercantile is good for lunch and just OK for dinner. Or not, whatever.
In any case, a selling point is that the restaurant also doubles as a shop, and sells relevant fooding goods that you can purchase. This is a great feature, as it combines my love for eating with my love for buying things. I didn't purchase anything either of the times I was there, but I appreciated the option.
On a sidenote, I thought that my favorite thing about this place would be the homemade ice cream. It turned out that my favorite things are the mason jars that they serve their beverages in. This made me so happy that I wanted to go home and put my drinkable liquids into mason jars and then drink it all.