Currently, it feels like the Terrex Solo is trying to occupy a middle ground between stocky/durable/stiff approach shoes (like the Five Ten Guide Tennie or the Salewa Mountain Trainer) and light/flexible/breathable approach shoes (like the Patagonia Rover).
I’d rather see Adidas either (a) stick with the synthetic/mesh upper and highlight the mesh, bringing down the weight significantly and putting an emphasis on minimalism and breathability, or (b) decide that they’re going to move towards warmth and durability and use leather/nubuck/etc. instead of synthetics.
I feel similarly about the sole: the Terrex Solo has a relatively flexible sole, and the shoe accommodates twisting and bending really well. However, the sensitivity of the shoe is relatively low despite this flexibility, largely because of the amount of its thick and cushy padding. Again, I think the flexibility is nice, but I’d rather lose some of that padding to shed some weight and reduce the profile of the shoe.
This would simultaneously address my last issue with the Terrex Solo: the large volume and high profile. The Terrex isn’t particularly light, weighing 14 oz in a size 9, though it’s not especially heavy, either. It’s on par with the Five Ten Guide Tennie, and even a few ounces lighter than the La Sportiva Boulder X. However, it is a relatively large volume shoe and takes up a lot of room in a pack or on your harness.
The comfortable nature of the shoe is a real strong point. But ultimately, the Terrex Solo wears like a running shoe, but just has too much weight and bulk to be used that way.
Fit / Sizing
The Terrex Solo runs true to size and fits great. I have a medium volume foot (for a size 13, anyway) and I found the Terrex to fit well without an excess of dead space. I have more room in the Terrex Solo than in the Salewa Mountain Trainer, and ever so slightly more than the Patagonia Rover. The Five Ten Guide Tennie is probably closest to the Terrex Solo in terms of fit.
While the Terrex Solo is a cushy, comfortable shoe, I think Adidas could improve it by putting it on a diet and trimming down the profile. It’d still be quite comfortable but less bulky.