I have previously tested a Skin S-Lab Salomon bag, but though it is ace, it's not big enough for runs which require full winter kit. Well, it is big enough, but stopping to get something out of the bag can be a little like trying to complete a Rubik's Cube in a blizzard. The Pro 15 Set has enough room for full winter kit, spare food, an emergency blanket / bag, and more besides. It also comes with an integrated, but removable, bladder and drinking hose. I have actually removed this as I prefer to drink from soft flasks in the pouches on the front of the shoulder straps. If you were to use it though it was nice to see that it is thermally insulated to keep it cold in the warm, and warm in the cold. Obviously, without the bladder in place there is extra room inside the main body of the sack. Also of note, it does not come with softflasks, but the holders are present so you may want to factor in the cost of adding these if they are your preferred method.
 
Without the bladder and hose fitted the sack weighs virtually nothing. Pretty much all of the sak that is in contact with your body is lined with a fine foam mesh which is surprisingly comfortable and breathable. There is no internal structure to the sack so it needs packing accordingly, without hard lumps, as the mesh is pretty thin. However, given that most of what you are carrying is soft by nature it's easy enough to keep it comfortable.
 
On to the features. There are a LOT of them:
 
There's a handy thin carrying / hanging strap on the top.
There are elasticated waist and chest straps. These fix onto a series of hard white plastic bars meaning that they are vertically adjustable. It’s easy to tighten or loosen on the move.
There's a teeny, weeny whistle attached to a loop on one of the shoulder straps. It's attached in such a way that it can be moved to other locations if desired.
On each shoulder strap there are an elasticated mesh pocket for softflasks, which are also big enough for a phone or compact camera. Below these are two larger mesh pockets suitable for gels, food, a collapsible cup or similar. And then below these are a pair of even larger mesh pockets large enough for a hat, gloves, buff, and other bits of clothing you might want quick access to. 5 of the 6 pockets  have elasticated top edges to help keep things stowed. The 6th one has a zipped top. I actually carry a grip top bag in this one and keep all my rubbish in there so I can't accidentally drop it. The latter two, biggest, pockets actually reach around to the back of the pack so are very spacious. The open topped one has a second opening on the back of the pack, the zipped one does not.
The main body of the pack is accessed by a two way zip with reflective plastic toggles. Inside there is a single pocket which rather frustratingly isn’t quite long enough for a standard sized map. The bladder access is via an opening between the main body and the mesh on the back. It is possible to slide a map into here, but it will have to be waterproof. The main body has ample room for spare clothes, and would probably even take a small sleeping bag and stove if needed.
In addition there is an elastic bungee with a toggle on the base of the pack meaning you could put a sleeping mat or bivvi bag on the outside.
At the top of the shoulder straps is a hidden toggle with a protruding elastic pull cord. These are for adjusting where the pack sits on your back, high or low. They are a little fiddly so it’s definitely worth getting them right before you set off on a long race.
There are also a pair of pull cord adjusters in the bottom hem of the pack allowing yet more fine tuning / fitting. 
And last, but not least there are a set of loops and elasticated straps for attaching a pair of folder walking poles.
The blurb says that it’s also easy to fit on an ice axe and / or a helmet. I look forward to the winter and trying this out!
 
Essentially with the number of adjustments possible, and the stretch material used throughout it can be fitted to your back and shoulders pretty much whatever size you are! I used this pack on the Glencoe Skyline having tested it out and set it up on a couple of practice runs in Snowdonia. The race took me nearly 12 hours and I am delighted to say that not once did it give me any discomfort whilst running, and it left no bruising or chafing afterwards. During the race if I needed to access the main compartment whilst on the move I unclipped the elasticated chest and waist straps, removed the sack, and put it on my front for a while. Otherwise all of the pockets were accessible without having to remove it. It retails at £130, but as with most specialist kit that I review, I think it's definitely worth the money if you use it properly. It's been designed by people who know what they re doing, with decent quality materials, and is definitely the right tool for the job.
 
More info on the Salomon website.