I've worked in this area for about 2-3 years and I've seen the market for his work go from naught to sixty in the space of 12 months and now its dropping off again becuase the initial buzz around it has faded away.
I reckon in the next 6 months you'll not be able to sell a Banksy for toffee, which is only fair really considering the only people that are making serious money from it are those who exploited the market in the first place.
I think it's pretty clever and actually quite like it, but having said that I've never seen any of his other exhibitions and never really followed any of his work to be honest! I was always under the impression he was 'underground' and wasn't interested in the commercial elements of art? Maybe that's just a misconception on my part? Do you know how many exhibitions he has done before now Lee? Do you like his work generally?