I bought the Extang solid fold, I looked at both the bakflip and the the solid fold.
I saw both on trucks side by side, I really liked the look of the bakflip with the flush mount, but I was not super thrilled with its lack of weather resistance (it was raining slightly the day I looked at them.) I also thought the Bak felt a little "cheap" for the price, just did not feel as sturdy or solid as expected.
The solid fold, was very weather resistant, very sturdy, and easy to take on and off, honestly can have it off and back on the truck in less than 3 mins max, and that is including aligning it to the bed, the clamps are quick to operate and very solid. It was just a better choice for me personally.
The Bak has some nice features no doubt! just not what I was looking for.