We bought the stretch seats for our upcoming July flight (our first on Frontier). After we purchased the ticket and the package that included the stretch seat we googled reviews of Frontier. We saw people posting reviews online that included some photos. There was one person that said they bought the stretch seat and it showed their knees right up against the seat in front of them. It said they were glad they bought it because they wouldn't have fit in a regular seat and he was only 6'2. (They also showed pictures that the tray table is tiny, like only big enough for your phone and a drink.)
We normally fly Southwest and after reading some of the reviews hope we aren't regretting flying Frontier.