Even secured inside a Pacsafe, I'm not sure I'd tie my backpack up to, say, a bike rack or a telephone pole and take off on a tour of Prague luggage-free for the afternoon, but I do approve of the eXomesh-cage-and-lock approach at the foot of a bed or leg of a bunk while sleeping in a hostel. Or a dorm shared with a particularly nosy or klepto roommate.
Pacsafe's anti-theft eXomesh cage systems come in sizes 25L and 35L for school backpacks, and 55L, 85L, 120L, and 140L for larger packs, duffels, and travelers' bags. The webs of protection secure shut safe and tight with a heavy-duty locking system, and their high-tensile, stainless steel wires are slash-proof to would-be thieves. Obviously, the mesh system has holes through which someone could slash the bag it holds for access to the bag's contents, but this seems like a pretty tedious and time-consuming way to steal someone's stuff, and in my experience as a petty thief, petty thieves do not have a lot of patience.
I suppose people could also knife through the Pacsafe mesh and cut holes in your bag just to be mean, but so too could they key your car or tell your girlfriend they saw you making out with her sister last Friday night. It's a risk you must determine whether or not you are willing to take.
Muchas danke to Bless This Stuff.