We just took delivery of the RED PRO Mattebox, and we bought it with the full cage/side flags kit + 1 top flag.
The mattebox itself gets a big thumbs up - lightweight, but feels very solid, and with a good build quality. Ok, it doesn't have a rotating stage for a polarizer, nor is there a tray larger than 4x5,65 (so grads are a no go) but it is what it is, and as such, we like it a lot! (one small complaint about the mattebox may be that the 110 and the 114 mm rings have a very nice twist lock mechanism - however, for 142mm (RED18-85 zoom) there is a more finniky ring that is not very easy to snap in place, and feels like it could break very easily = no easy switching between 142mm and the other sizes)
Hovever - the cage/side flags top/bottom flags are NOT GOOD. It's not a problem with the build quality, it's a problem with poor design, which feels like an armchair/desktop design, without proper input/feedback from actual production work.
The basic issue is this: on for example a Chroziel matte box, you would tighten the locking screws for just enough friction so that the flags would stay put, but you would still be able to adjust them up/down/side. This is not possible with the RED PRO Mattebox cage. If you tighten the screws so that you can still adjust the position of the flags, any small amount of adjustment will unscrew the locking mechanism leaving them flopping like my golden retrievers ears = BAD. This may not have been a huge issue, had the position locking screws been easy to tighten... they are not. Depending on the size of your hands, they may be next to impossible to tighten - especially the top flag ones without swinging the side flags out of the way. And, I don't even want to think about doing this in cold weather with semi-frozen fingers. And, on top of it all, there is not ONE screw, there is TWO per top/bottom/side. And, a further complaint about the design of the side flags is this: the expansion locking screw is on the FIXED part of the side flag, it should be on the MOVABLE part, doubling for a "handle", making a one handed adjustment easy. As it stands now, it's finniky to do even with two hands. (And, like with the positioning locking screws, they shouldn't really be "locking", but rather a tool to dial in the right amount of friction, so that you can just push it to the correct position, and have it stay.)
In summary - the mattebox it self (with it's known limitations) is a big thumbs up, the cage and flag system is a big thumbs down. Cage and flag system seeds to be re-designed.
My 2 cents.