I have the 120-300 EX OS HSM and I really like it.
On the plus side,
- cheap compared to other 300/2.8 (plus it zooms!)
- sharp (considering how cheap it is)
- takes 1.4 extender nicely if stopped down to about f/5.6 or f/8.
On the down side
- not as contrasty as for instance the 135 /2 or the 70-200 /4 L IS (I can only compare to other lenses I own, I bet the 70-200 /2.8 L IS II is better aswell). It kind of lack "magic" but produces good photos.
- not so sharp in corners wide open
- not so sharp in corners @300 mm
But considering the fact that you get a unique lens... I´d say it is totally worth it, even the non sport one (that I own).
Ehm, photos.... well I mostly use it to shoot agriculture sohttps://www.flickr.com/photos/marten_svensson/15235254360/in/photostream/https://www.flickr.com/photos/marten_svensson/15235186729/in/photostream/
Anyways... I like it. As always, use it where it shines and you will get great results.