I have to concur somewhat. I've blocked major chunks of AGIS' address space,
and 22 networks worldwide now mirror my personal black hole list, and though
I receive periodic forwarded complaints from folks who can't reach someplace
which shares address space with a spammer, noone has EVER complained to me
at not being able to reach a blocked AGIS customer.
AGIS proved this case by heavily restricting their peering policy a while
back. Several companies I know of chose not to seek transit just to reach
AGIS, and so far none of them has felt any pain at all from their decision.
I'm going to stop now since those are the operational/connectivity issues and
I don't want to grab your attention and then start ranting about AGIS's "new"
opt-out formulae, or about spam in general.