do you use SPF TXT RRs? (RFC4408)

A partner had a security audit done on their site. The report said they were at risk of a DoS due to the fact they didn't have a SPF record. I commented to his team that the SPF idea has yet to see anything near mass deployment and of the millions of emails leaving our environment yearly, I doubt any of them have ever been dropped due to us not having an SPF record in our DNS. When a client's email doesn't arrive somewhere, we will hear about it quickly, and its investigated/reported upon. I'm not opposed to putting one in our DNS, and probably will now - for completeness/best practice sake.. how many of you are using SPF records? Do you have an opinion on their use/non use of? take care, greg