The IQ is fantastic on the f4 L IS, I've compared it to the 100 macro and 45 TS/E and it's just as good. Edge to edge at f4 is great, no real IQ gain stopping it down. Copy to copy variation seems to be great as well, have yet to hear of anyone that got a bad one. The 70-200 f2.8 I had was soft at f2.8 so I just never shot it at that. One more thing the f4 L IS is great with 1.4 TC's.
I've owned both the 24-70L and 24-105 L and I'm a big 24-105L fan. If you need f2.8 though obviously the 24-70 is a great lens - just kind of clunky. If you need a wide angle to replace the 17-40 I can recommend the 17-35 Tamron f2.8-4, here's a shot from it. It's not bad for a $270 lens. (sorry same old shot I know!)