Surface mounting, pretty much. I agree, it is a lot more expensive than an average trigger lock (last Master combo I bought was about $15), and you can buy cheap safes for that amount, but the benefit is the ability to mount the lock anywhere, in any position: Wall, closet, under the bed, behind the seat of your pickup. From what I understand, you can unlock your shotgun pretty quickly if you need it (although it still needs to be loaded).
Still, it's personal preference. If you can use it, great. If not, don't bother. I keep all my firearms locked in their boxes, but I just bought a fourth and it's starting to take a bit of room, so right now a safe makes more sense than a Shotlock. $100 on sale, if I buy a cheap one at Can-Tire, although with a bit of effort it should be more than adequete.