It was time to move up from my 70-200 F4L and the older f2.8 version's were not an option due to their week performance at f2.8 so I bit the bullet. So far this lens almost stays permanently on my 5D MKII and it has earned it's money back several times over.
Very very sharp; Too sharp actually at times; weather sealing; nice new locking hood design; Improved IS over older versions; can really hand hold to 3 real stops under; 3D like rendering of subjects in circle of confusion ( Almost Pentax limited lens like; great colour rendering and contrast,...
PRICE; I don't know of Pentax patented it but a hood window to adjust polarizing filters or similar would be nice; Strange bokeh; it's a bit harsh with back ground high lights and can make some hard yet still round halos.