I've never heard of Banksy until this, just took a look at the website and I think all those images on public property are fantastic!
I'm biased though, when I was a teenager, one of my good friends was a Hip Hip style Graffiti artist and of course had to do all his stuff illegally but in similar places. More dangerous but 'street cred' places earned respect of others who did similar, back then, they took photo's as soon as the piece was finished as it usually was a matter of days before the council would paint over it.
Some of his work was totally mind blowing, 20 foot long and 8-10 foot high spectacular graffiti art completed in one night!
It got to the point he had a meeting with the council and was 'given' a wall to graffiti on! I think it was something like over 6 months he could change it 3 times so long as he told the Police in advance he'd be painting there!
He'd still do his London Underground pieces as well and of course make sure he wasn't caught, it's unfortunate that British Transport Police didn't look on it as art, rather as vandalism - he lost 2 of his friends being chased by the transport police as in attempts to get away they touched the live 3rd rail.
To me, most definitley, when graffiti is done like this, it's Art - no question - and this Banksy has a great socio political satire/slant on many of the pieces - Great stuff!
I only ever got to the stage of getting a A3 piece of paper and being able to copy/sketch stuff - I tried to learn how to use a spray can, but didn't quite get it !
Cheers, DtM! West London & Slough UK!