Surprisingly good given the size and price (and the surprising distortion correction compared to - say - the new Tamron primes which are reported to be breaking negative records in this regard despite being of narrower angle and physically larger. Good to have an f/4 lens with an emergency f/2.8 capability, too. 
Oh, and looking at the block diagram, it's funny that this lens only has one regular glass element, the other 8 are all either aspherical or made of special glass. Maybe that's how the manufacturer was able to cram so much performance into such a tiny package.
Oh, and looking at the block diagram, it's funny that this lens only has one regular glass element, the other 8 are all either aspherical or made of special glass. Maybe that's how the manufacturer was able to cram so much performance into such a tiny package.