Don't mess with DNS
Intercepting ones browsing session in order to insert adverts is the height of bad manners. Especially when you're paying for the experience.
OpenDNS do the same thing, but to be fair to them, it's in lieu of providing a free DNS/content filtering service (if you're that way inclined you can block their ad page at the router, giving you the original "failed" ad-free experience).
Biting the hand that feeds IT
