I had the 17-40, sold it, and bought the 16-35 on sale. i ended up paying about $400 to upgrade.
IS is great when I am using it as the walk around/standard lens on my 1D4 (it feels like a low 20s to 50 zoom on the 1.3X crop).
But, I am not so sure the sharpness is a big enough issue alone if you are using it for landscapes, on a tripod, at f/8 or f/11.
On my 5D3, it certainly does look somewhat sharper in the corners, but it's not the major major difference many imply when compared to the 17-40.
5D3,1D4,S90,6S&Moment Lenses,Hero4Silver,GPS-E2,2x580EX,430EX,90EX,EF16-35L f/4 IS,Samy SYTS24-C 24TS,EF24-105L IS,EF50 f/1.4,EF70-200 f/4L IS,EF300 f/4L IS,EF100-400L I IS,Kenko DGX 1.4X,Canon 2X TC Mk II, RRS&Pro-Media L-Brackets,Manfrotto MHXPRO-3WG & Roller 50,Sirui 306&K-20,Giottos MT-7371&MH-3300,Velbon ElCarmagne 530,CamRanger,Phottix&Canon Remotes,Lowepro Backpack,ThinkTank Retro 20&Modular System,OpTech straps,Lexar/San Disk Cards