You get what you paid for. I recently sold my 20-35 f2.8L which IMO was already a very good sharp lens with excellent build quality.. unfortunately, this lens lost its ability to switch from manual to auto (due to cracked switch) and since I am a wedding photographer, autofocus is not something I cannot do without. Now, the 16-35 replaces my WA focal length. So far, this lense has matched and exceeded expections. On my 1dmk2, ~20mm-46mm equivalent, perfect for my weddings.. from wide to 'normal' perspective.. and on my 1N.. hehehe.. simply wiiiiiiide...
However, I would not recommend this lense to an enthusiast.. value for money is not there. This lense makes money for me as a professional, hence, justifies its purchase but i simply cannot see myself as an enthusiast purchasing this lens.. just no way to recoup the investment.
my 2 cts.