I feel like the Elephant in the room saying this but Food Truck Friday in Phoenix sucks. The experience has disappointed me every single time I've been.
Don't get me wrong, I love supporting the small guys and will happily pay for great food but the value proposition of FTF is terrible. I guess it's the hipster culture that commands the high prices but it doesn't excuse mediocre food.
Price: Don't expect to leave on a full belly with any less than $20 in your pocket.
Quality of food: I know this solely is on the vendors but I've yet to find a vendor that sells anything that wants me coming back for more. ALSO don't be a fool like I was and buy something based on the ingredients listed - make sure you SEE what you're getting first.
One time I basically paid $10 sight unseen for what amounted to a side of curry and rice from a vendor (I hate Indian curry and nowhere on the menu did it sound as if it were a curry). I was very not happy. There was "Cambodian" something in the name which sold me on the "dish" (I am Cambodian). Believe me when I say it was not Cambodian in any way shape or form, not even the rice was right.
The burger vendor is ok, it took forever and the meat was on the verge of rubbery, but at least it was filling.
Short leash is probably the best I've had, they are not bad in the taste department but you are paying $8 for one fancy hot dog all said and done. No offense intended but they also serve their hot dogs in pita bread. They call it Naan so they can charge you more for it and sound culturally diverse.
Ambience: It's like a Carnival, and that includes the crowd it attracts and the cleanliness. Consider yourself lucky if you can find a seat anywhere, let alone a clean spot. The parking lot is free I think which is super nice for downtown Phoenix. It is a crappy rock lot though so don't bring your fancy car if you can avoid it.
In conclusion: Do it at least once maybe just for the experience. If you want to feel like you are expanding your horizons or something go ahead, waste $20 for mediocre food. I'd much rather eat most anywhere else though.