According to the Pierre Sprey, co-designer of the F-16, the F35 is a turkey. Inherently, a terrible airplane. An airplane built for a dumb idea. A kludge that will fail time and time again. Just impossibly hopeless. And judging from the bajillion times the F-35 fleet has been grounded, well, he's probably not wrong. It's a trillion dollar failure. Watch Sprey eviscerate the F-35 in the video below.
Image Credit: U.S. Air Force photo by Jim Hazeltine
Update: Here's the extensive and intensive rebuttal by Foxtrot Alpha Tyler Rogoway, in which he says that "half of what [Sprey] says is totally relevant, the other half is totally bullshit."