1. It does not have the reach I need for surfing and wildlife pictures (even on a crop 7D)
2. You cannot use a 1.4 or 2.0 extender.
3. It does not come with a tripod collar ($185)
It is a great walk around lens, especially for street photography. No issues with IQ. Just does not have the reach for me.
... see the last 70-200 and 70-300 during the last month, too, there's all information and many opinions in it. My 2 cents:
* global impression: I've got the 70-300L for about 3 weeks and it's my primary walk-around lens now because 70mm is good enough so that I can zoom with my feet and take in most *objects* I'm interested in. For me, the size-weight-iq-af-zoomfactor-buildquality-price combination and tradeoff is just right and I'd recommend it for everyone who wants to spend the cash (and not double or half) and wants this zoom range.
* correction: You can put on a Kenko 1.4tc, I'll get it myself since the iq is said to be still ok - but af is slow at f8 (you have to trick the camera into thinking it's actually f5.6)
* negative point: on ff it's said to have less iq than the 70-200s in the corners and might show vignetting (easily corrected by software of course). But the 70-200/2.8is2+tc is double the price... and since I'm often shooting @300mm I'd hate to put on and take off the tc all the time.
* negative point: no tripod collar (thanks, Canon!) but not really required since the lens is short and not that heavy so there is less torsion on the body in comparison e.g. to a 70-200/2.8
* negative point: extending zoom, I'd like a internal zooms better, but this is the tradeoff for the short form factor.
* negative point: no zoom range limiter like the 70-200, but af is very accurate and fast it does not matter that much except for low light when the af might start to hunt.