There are many alpine rock routes including technical rock climbing, that you want to change into rock shoes for, that you need to carry the shoes you approach in. This is the category this shoe works brilliantly in, that I call approach shoes. Low weight is key here, you don't want to lug your old heavy leather tramping boots up a hard pitch, but they also need to be robust enough to handle a scrambling approach or scree descent that might rip a pair of light fabric trainers to shreds. The Salewa Firetail Evo is my favourite shoe in this category. They were light enough to carry up a hard pitch of a new route at Cloudy Peak, and also robust enough to handle the long scree descent.
I also have extensive experience using the Salewa Mountain Trainer which is worth comparing with. A pair of Firetails (375g/shoe) saves me 340g compared to the MTN trainer (545g/shoe) in my pack. Wheras the MTN trainer felt like a low top boot, with leather upper, stiff midsole and chunky, bootlike tread, the Firetail is closer to a running shoe, lighter, softer with a thinner, sticker tread. The upper is a synthetic fabric. It's not as solid as the leather MTN trainer, but is more robust than a light fabric trainer and has lasted well scrambling on rough Darrans granite, or sharp Canterbury scree which has previously shredded lighter shoes in a single descent of Cloudy Peak.
They have been ideal for Darrans approaches involving mostly rock slab, and have been up the Nth Buttress of Sabre Peak and the traverse of Marian and Barrier Peaks, and the approach to Barrier knob and Moir's Mate several times. This included short snow sections, for which I've paired them with Camp XLC490 Universal Aluminium Crampons, a combination which has worked well. This combination weighs (810g + 607g = 1417g) compared to a light summer boot plus steel crampon combination (for example Salewa Raven Combi ,1600g plus BD Sabretooth 1158g = 2758g), making the approach shoe-aluminium crampon combo check in at half the weight.
This has big limitations though. While ok on short, softer snow, it would be dangerous on ice, and uncomfortable on sustained or steep snow travel, so use with great caution! The Firetail Evo climbs easy rock and scrambles well. I'd use it for something like the grand traverse of single and double cone which involves some technical scrambling, and gives great grip on slab approaches in the Darrans. It is a softer, more comfortable shoe for long trails than the MTN trainer for trails and tramping and is ok for some running, which I'd avoid with the MTN trainer. For longer, off track travel involving more snow, or steep grass I'd prefer a stiffer shoe with chunkier grip, such as the MTN trainer. Also the MTN trainer is more robust and will last longer. For this reason I took the MTN trainer to Patagonia for approaches on sharp glacial moraine and also for standing in etriers in Yosemite.
But when weight matters, especially when free climbing carrying the shoe, for example in Darrans, Bugaboos or other similar alpine rock, the Firetail evo is my top pick. Fit wise, I was surprised to find them size a little differently to the MTN trainer, and take a size down. They handle my wide forefoot well. They come with a multi-fit inner sole system which is a great way of fine-tuning the fit, I use a volume reducing inner sole to reduce movement. The outside of the forefoot eventually blows out with extensive use, reinforcing with shoe goo in this area is recommended.