Until spf becomes the standard it will be difficult to see how effective it really is. The use of spf (Caller-ID MS ? Similar) is a step in the right direction to hold spammer accountable for their actions. Adding spf txt records should not affect mail delivery.
It is only used when the records exist.
Again spf is not a stop all spammer solution. There are steps once the spammer is identified that have to be taken.
SpamAssassin (configured to use the sbl-xbl.spamhaus.org DNSbl) + DCC + Razor (optionally) is currently the best way to filter spam on a Unix system, in my opinion. I think that newer versions of SA either have, or will soon have, SPF support.