My wife and I found this little gem when we moved to South Phoenix, and I have to say it's the best Thai food I've found in Phoenix, and nothing else is even close.
I typically get cashew chicken. They offer 5 levels of spiciness for their dishes, and I absolutely LOVE spicy food, so I go for a 5 every time. Let me tell you, it does not disappoint when it comes to flavor or spice.
My wife, on the other hand, typically gets royal curry chicken, with a spiciness level of 1 (she can't even handle the spice of black pepper). The curry is wonderful by her account (she loves curry) and also by mine (I have never been fond of curry until trying Chanpen).
It's often the holes in the wall that house the greatest treasures when it comes to cuisine, and this is no exception. If you like Thai food or even if you don't, you owe it to yourself to try Chanpen. It's the best Phoenix has to offer.