i had a 17-40L f4. great lens and it was my favourite lens until i bought a 16-35mk2 L f2.8. it's twice the price, not necessarily twice as good, but i love it to bits. if i did purely landscapes, i'd buy a couple of L primes instead of this, but the zoom suits what i do, and i don't do enough landscapes to justify the extra expenditure.
get the mk2 if you plan on getting one.