It's a great way to get in some conditioning work when you are on a strength and/or size focused training schedule and can't afford to do too much sprinting or other high intensity or high impact conditioning. The best use for it in my opinion is to go hill walking - do not underestimate how good a conditioning tool this is, it will crush you. Just make sure to use at least the 40 pound one otherwise it is not worthwhile.
A good backpack is probably okay to use for general conditioning but I would make sure it is an appropriate backpack designed to bear weight over long distances. Also worth to note that a proper weighted vest distributes the weight all around you trunk meaning your upper back/traps are much more involved - the backpack primarily loads your lower back only.