Love this idea. I have been wanted to check this out for awhile now. great location in South End. Probably 8-10 different trucks with lots of variety. The best part about this weekly event is the diversity in people you see. All ages and ethnicities can be found here. Lots of dogs.
As for the food, I tried the Tin Kitchen which was amazing. Had their chicken mole taco and a duck confit with sweet potato puree taco. Both were very, very good. On the side I tried the 3 bean Peruvian salad which was also just as great. My wife tried a fancy grilled cheese from Papi Queso- excellent.
My one suggestion would be to get there early (by 630pm) and bring some chairs or a blanket. Common Market is located right next door so you can grab beer/wine there if you don't bring your own.