I owned the 70-300 IS and was looking for more reach. Didn't want the cost and physical size, so I started thinking about a better IQ and cropping.
Went down to the local store and tried a 70-200 2.8 with a teleconverter and the new 70-300 L. Low light, indoors, taking a picture of a picture - WOW. I have the new 70-300 L in my bag now. It is tack sharp edge to edge on my crop sensor (60d). I had no idea what the 60d was capable of until I put this lens on.
I was in no ways impressed with the 70-200 II 2.8 with the new 2x converter (didn't try the 1.4x) at roughly 300mm zoom. Handheld, it underperformed my old 70-300 IS (didn't bother trying the tripod because I don't get to shoot with the tripod much which, yes, disqualifies me by the standards of many folks).
I'm just a hobbyist interested in some wildlife and landscape... I had no problem seeing the value in the upgrade. Only problem is now I want more L glass (I think the L stands for Lust).