I loved the Montrail Rogue Racer shoe. It provided a decent amount of cushioning in a very lightweight shoe. The Rogue Fly is the next step. As far as I can tell, the Fly keeps the exact same sole as the Racer, but provides an even lighter upper.
Does it work? Absolutely. My size 10 Flys weigh 460g (pair); my Rogue Racers 505g. That might not seem like much of a weight saving, but it's still 10%. And so far, I've not missed the lack of structure in the uppers. I could be imagining things, but it also seems like the Flys run a little narrower than the Racers.
What I still don't like is the amount of heel-to-toe drop. (10mm, I think.) I do like that the heel is well-cushioned, as it means I can run fast downhill. (Overtaking runners wearing really minimal shoes!) But I'm not sure I need this much heel height, nor if it is really appropriate for such a lightweight shoe. I think Montrail could decrease HTTD to, say, 6mm, and further lighten the shoe without damaging its performance. I have no problem running fast downhill in Brooks Pure Grit, with a 4mm HTTD.
Durability may be an issue. As Montrail shave more and more weight off the shoe, something has to give. I think the product description should emphasize more that this is really a stripped-down racing shoe. However, my Rogue Racers have lasted through many long races and lots of abuse on the trails, so hopefully the Flys will last similarly.