Moderately popular in the uk, a bit shit tbh, better off with other brands. Had multiple pieces of kit from them and consistently been disappointed, whether it's design, or poor build quality, a lot of stuff that makes you think "why did they do that?" or "was this not caught out in wear trials?". One friend had a waterproof jacket that had regular coil zips for the hand pockets no storm flaps no funky drainage system just flat out bad design.
I've also caught them out with some pretty outrageous claimed product weights. I have a friend who works at a prominent gear store, they said returns for quality issues were pretty much the same across the board, except for montane, montane had more returns than other technical brands. E9 quality, is also pretty poor apparently.
This is all quite a shame, I want them to be good the specs are good, a lot of the core fundamentals are good, but they just get the details wrong.
If you want a fast and light brand that is mountain running oriented I'd recommend OMM if you can get it in the US. Rab is sold at a similar price point to montane these days and generally has more compelling products though they have certainly had some mind bogglingly bad products in recent years.
Mountain Equipment is where its at but you pay for it.