The price above is given in USD but was acutally bought in the UK for about £4000 (gbp) - 2:1 exachange rate at time of writing.
I do a lot of walking and end up taking a lot my photos hand held, so i wanted something light and easy to carry up a mountain along with camping gear (already 40+lbs for a 3 or 4 day hike) without breaking my back, with IS and around the 400mm mark.
Camera bodies are similar price in the UK and US, but the big primes seem to be about 60% more expensive here at the moment (no idea why), so I agonised for ages over buying this lens due to the frankly ludicrious cost of it here.
I'd been using the 100-400mm IS for a couple of years due to it's light weight, but was never happy with the sharpness or (at times) contrast. Maybe i have a bad copy, but it just had to go - i took some lovely shots with it over the years, but i always lamented the lack of sharpness in them compared to my other lenses.
I thought about the 300mm 2.8, but i knew from having to add my 1.4 to my 100-400 plenty of times that i'd end up buying a 2x teleconverter and be no better off (quality wise) than i was before.
All the mixed reviews about this lens made it really hard to decide - it's a LOT of money to gamble when some have said the 400 DO is no sharper than the 100-400.
Well I eventually bit the bullet and bought one... and oh my god. It's like night and day. Going from my 100-400 to this thing is probably more of a 'wow' than when i upgraded from a 300D to a 1D Mk 2.
The sharpness is absolutely excellent (including with my 1.4 teleconverter) and so far the contrast and colour has been spot on as well.
The AF is also much, much faster than the 100-400 and the extra f stop over the 100-400 (at 400) makes a difference too. Getting up to 1/1500th of a second at iso 100 (in april in england!) certainly puts a big smile on my face.
I don't know what QC problems canon had with this lens when it first launched but if my copy is anything to go by they're gone.
Finally i have the lens i'd hoped my 100-400 would be when i bought it years ago - light enough to hike with and hand hold and fantasic images without having to sharpen them to death in PS afterwards - i just had to pay 4x as much for it and loose the zoom!