SPF has problems
I thought I was doing a good thing by adding SPF to my email server. Unfortunately, I am now finding that there are some serious problems with this scheme. The biggest one has no solution (that I know of) other than to disable SPF: If your ISP forces you to send email only through their SMTP server (lots of ISPs do this now), then your emails will all fail the SPF test because they originate from your ISP's SMTP server instead of your own. This also occurs when you are travelling and hook up to a random WiFi hotspot. Many of them funnel email through their SMTP servers.
Someone's written an article about these types of problems. He's a bit extreme but has some good points.