Yeah, it's the same product molded into different shapes at BK and Subway (at least in the US). I used to have the joy of working for teh company that produces them.
I second the Red Robin recommendation if you have one near you. (Can't offer an opinion on Denny's, as I haven't been in one in over 20 years.) Not sure if it's chain-wide, but we have a choice between Boca Burger and Gardenburger at our local RRs. The menu appeals to omni's too, so it's a great place to suggest for a group feast. I've also run in there for a to-go order a few times rather than going the fast food route.