I have SPF verification enabled on my mail server. While SPF is no panacea for the problem of spam, it is quite effective at ensuring spammers do not forge the sending address to impersonate someone else, and cause some poor innocent soul to receive in a boomerang effect the torrent of complaints hurled at them.
Unfortunately far too many lame organizations (cough, Google) qualify their SPF record using a too permissive
~all clause, which means they have servers other than those listed, and thus their SPF record is useless for filtering purposes.
In the last month, I noticed the opposite problem: I did not receive emails from Eurostar and BookMooch because their SPF records did not list the mail servers they actually use. If they are not clueful enough to manage a simple list of IP addresses, or have basic change management discipline, they should do us all a favor and ditch the SPF record they clearly are incapable of maintaining.