The bottom line: The Rab Spire Pants are softshell technical pants made for mountaineering and ice climbing. They are incredibly comfortable and stretchy, and durable enough to stand up to tough conditions. They're a bit overkill for hiking, but they still work great in the winter for a bit of extra protection from wind and snow. They are very windproof, but are loose enough to fit another baselayer underneath. Overall, these are fantastic pants at a pretty reasonable price, and I would highly recommend them.
As I started getting more interested in winter hiking I wanted a pair of durable hiking pants that would keep me dry and comfortable. I settled on the Rab Spire Pants because they were reasonable priced, came DWR treated, and had a slim fit that I liked. Instantly when I put them on in my room I could feel how comfortable they were, and I wore them all night around the house because they were so comfy!
The Matrix softshell fabric is very stretchy, which helps with the comfort as well as with climbing flexibility (no need to worry about those high steps when hiking or climbing!). This is one of my favorite features of these pants, and the primary reason I love them so much. The inside of the fabric is slightly soft, although I usually am wearing a synthetic baselayer underneath these pants. The waist is also fleece-lined for extra comfort. The removable belt has a plastic buckle and is easily adjustable. I found that the pants fit me well, although they are large around my waist, even for a slim fit. The integrated belt helps but is still a bit small. Rab has fairly detailed sizing charts, so I'd recommend reading those closely before buying.
The Spire pants have two zippered hand pockets as well as a zippered thigh pocket. As with the construction of the entire pants, the zippers are solid and sturdy. (I did have to retie the zipper pull once, but the zipper itself is high quality.)
There is reinforced material that covers the knees and inside of the calf, to protect against crampons. The bottom of the legs also have a short zipper and gusset to help pull the legs down over your boots. Note that you won't be able to pull them on while wearing your boots though.
These pants are pretty breathable, and I've never had any trouble with them retaining moisture as long as my baselayer pants are appropriate. The Spire pants are medium weight in terms of insulation, so they won't keep you too hot or cold - but I found them to be great at blocking wind (which is the primary purpose of a softshell anyways). As I mentioned, they are DWR coated - which works well in very light precipitation, but won't protect you in rain. (Of course, these pants will mostly be used when it's too cold to rain anyways.)
Rab say that these pants are meant for "alpine climbing and other fast moving mountain activities." In my opinion, for less intense or warmer winter hiking, these pants are overkill (though definitely still usable and useful). For summer hiking, I would definitely choose a lighter pant. But where these pants really shine is for more difficult/intense winter hiking, colder/snowier/windier conditions, and mountaineering/climbing. For those uses I highly recommend the Rab Spire Pants.