Rating: 4 / 5
This is the newer version of the Prumus/Brunton Crux canister-top stove
and appears to have corrected faults reported by Crux owners.
First the number of fold-out pot supports has increased from 3 to 4, for greater pot stability.
Second the "wobble" of the retracting sleeve that permits the burner to be folded 90 deg. no longer exists in the Flex. Evidently tighter tolerances are responsible for the cure.
My reasons for buying the Flex (which is 0.7 oz. heavier than my Vargo Jet-Ti burner) is that it has a much larger flame ring, making for more even heating and more heat,should it be required. Also the Flex has wider pot rests for my 1.5 L. JetBoil pot. I have no JetBoil stove but bought the pot for its greater fuel efficiency when compared to other pots. A wider pot rest is nice to have with a wider pot base. Further, to enhance stability I recommend getting Brunton's three leg canister stand. It's great when setting up on uneven rock for cooking. Three legs always are better than four legs on uneven surfaces.
The relatively large Flex pot rests still fit inside my JB pot's corrugated "Flux Ring" which circles the pots' bottom. This ring is the heart of the JetBoil system.
So, while I'll be carrying barely 3/4 oz. more weight I'll have a better stove for that little gain in weight.
One recommendation I would make is that Brunton have some sort of indentations near the hinge area of the pot rests to create a friction lock so the pot rests "click" into place. They are too floppy as they are. A mating "male/female" indentation system would be nice, and easy to do during fabrication.
And the final recommendation to ALL lightweight canister stove makers (MSR Pocket Rocket excepted) it to put a "Y" shaped piece of vertical sheet metal right on top of the burner to prevent wind from blowing out the entire flame. Coleman has done this for years on their big camp stoves and the Pocket Rocket has it as well.