The Tibets are legit stiff sole boots. If that is the type of boot that you are looking for, they are great. Once you get some miles on them, they will get a little bit of flex to them, but not a ton. With a heavy pack, the Tibets really earn their keep. Another option is the Lowa Ticam II boots. They have a sole that is very similar to the Tibets, but they are not as 'blocky' in the toe area, and they are a bit lighter than the Tibets, but they are still plenty rugged and supportive. I really like both of these, but I may be starting to like the Ticams a little more.
As far as heel slip goes, the Tibets and Ticams do a great job of locking my heel in place, so I get no heel movement at all.