As some people know I did spend quite a long time (back when I practiced a LOT and tried everything to improve, minus Hanon...ick) with the Liszt exercises and have again and again told the same story about them on the forums. As technical exercises go, I like them. I haven't used them very often for a few months, but I like them and if I decided to go back to working on exercises for some reason, I'd do them. Let me put it this way: as far as pure, raw technique is concerned, I am convinced that if someone is capable of practicing all of the exercises 100% correctly, he is both supremely achieved technically and also doesn't need to do the exercises; he would be "unstoppable" as far as technical abilities go.
For me, the largest advantage of separate exercises as in the Liszt book is that one can work on technique, obviously an imperative part of piano playing, apart from music, which in itself is extremely demanding in most cases. Someone, like me a while ago, who is not fully able to concentrate on both things at the same time (but is nevertheless a significantly advanced player), will clearly benefit from having separated both halves of piano playing and making technique a second-nature, microsecond reflex and concentrate entirely on music alone when he works on the repertoire.
On the other hand, it DOES require a large chunk of time, as Alde mentioned, and also a lot of energy as the pianist must always be focused on technique and every minute motion and position of both hands (actually, I prefer to just do one at a time) and so forth. Tension must be monitored at all times, as well, all over the body.
You can certainly make the exercises work for you, though, as long as you do them all properly. And because they are SO demanding, sometimes presenting realistic or absurd technical demands (mentioned earlier), they will equip you with extremely advanced technical knowledge.
But, at the same time, they are not necessary and they are not for everyone.
Topic: Liszt technical studies