The pack is designed to work with the Salomon custom system. It states that it is compatible with all XA series of packs (I have the XA 20) but honestly, the Velcro strap attachments can be rigged to work with just about any pack, as my team mates can attest to.
The pack adds 5 liters of volume to your current carrying capacity, but the major benefits include ease of access for your commonly needed items (compass, maps, nutrition, etc.). Having them up front saves time and energy, and when you’re dead tired that’s a priceless commodity!
Another great benefit is that it balances out some of the loading of your gear. Instead of having 100% of the weight on your back, you are able to front load a portion of that weight up front, counterbalancing (to an extent) the strain on your back. It took me a few training runs with both front and backpacks loaded with gear to get used to the new balance (works your abs more) but once you acclimatize to it, the benefits are significant.
The pack features two bottle holders with an elastic mesh electrolyte tube holder attached to one side. A triple layered mesh on the front for storage of paper or very small and thin items (maps, CP punch cards, etc) with race bib number clips. The main compartment is very expandable and offers the majority of the 5L capacity. This is great for holding your nutrition items you might need regular access to, or other gear you frequently utilize during the race. When the front compartment is not fully utilized it sits close to the chest and does not provide any obstruction to movement.
The back of the pack has the Salomon honeycomb air flow padding, which aids in air movement keeping you as cool as possible. The attachment straps are fully adjustable to ensure that the pack sits comfortably no matter your torso size, and comes with loops on the bottom rear of the pack to allow your backpack waist straps to connect through ensuring the pack does not move around when running/biking/etc.
Overall I highly recommend this addition to your standard backpack especially if you require the extra storage for longer races. The overall quality of the material and stitching is fantastic (as expected from a Salomon product).