Of the lenses I have I always said if the 2nd version of the 100-400 came out I would upgrade. Obviously, only if I saw image improvement. Well, I've seen the improvement and finally saved up. I still have the original. The decision now is whether to do it now or see if Canon offers a rebate on their next go around.
canonpricewatch.com has the new 100-400 II for $1999.99 in their street price program. It's an authorized dealer, not grey market.
That saves you $200 over the big guys in NYC. I've purchased several high dollar items through canonpricewatch.com and its been an excellent experience.