I used a Porroflex on a Mamiya C-22 years ago, and I also adapted one to replace the hood on a Minolta Autocord. I've shot with a Rollei equipped with the Rollei prism finder. The Rollei prism is brighter than the Porroflex, but the Porroflex is a lot lighter in weight. For shooting weddings with flash they allow you to use a bracket that centers the flash directly over the lens.
All but the oldest Rollieflexes (and the Rollieflex T, a simplified less expensive model) have a flip down mirror in the hood with a little eyepiece lens located just below the hood's rear cutout for using the sports finder. Look through that little lens and you can focus an upside down image in the center, then move your eye up about 10mm and frame with the sports finder. If you can find a Rollie hood from an F or G it'll fit on a T. The attachment thingie is the same.