This lens followed on from the EF17-35L (which I have). The early 16-35's exhibited some problems with peripheral softness and flare and were generally a step backwards. The MkII was introduced after a fairly short time to address these issues (successfully).
17-35's can usually be obtained for about 60% of the cost of the 16-35...well worth a punt.
There's not really an upgrade from the MkII but it may be that people are going from MkI's to MkII's.
Bob
I did a quick test between the MKI & MKII here
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.