Call me crazy but I thought the overpriced 24 and 35mm EF primes that came out with IS were a waste of time.
The 24 IS and 35 IS lenses are both spectacular for different reasons.
First, they replace older designs that were far inferior optically.
Second, the 24 IS has significantly less flare closed down (where most landscapers would use it) than both the 24L II and the 24-70 II. The smaller size also makes it more portable for hiking.
Third, the 35 IS has rounded aperture blades unlike the 35L, plus it is smaller and lighter than the 35L. Fantastic for a city walkabout lens for those reasons, plus less obtrusive/expensive looking.
Fourth, these lenses have the added bonus of IS, which can be useful in select circumstances when you lack a tripod. IS is really only a bonus though, the real beauty is in the other advantages mentioned.