All the more reason SPF is useless as an anti-spam tool. Users have no control over whether their ISP is implementing SPF, but the ways in which they are allowed to send legitimate mail are affected by their ISP's choice. Most users probably wont even know whether they are supposed to comply with SPF's restrictions.

SPF is an tool for preventing Joe-jobs --- it has some use in preventing spammers from forging the return domain on their mail (leaving aside the issue of DNS cache poisoning that can be used to evade that). But it doesn't do anything to prevent spam per se -- this article explains why. Indeed, research has shown that more spam domains are using SPF than non-spam domains, because spammers are much more motivated to get around aggressive anti-spam techniques than non-spammers are. As an anti-forgery tool, SPF doesn't do anything to address what makes spam spam -- its unsolicited and bulk nature. And it has serious follow-on implications for email architecture that have never been adequately addressed.

The fact that average users have no control over whether or not they are expected to comply with SPF just makes the problem worse. An antispam measure that 1) doesn't work, and 2) makes life more difficult for non-spamming end-users (as you admit) is not an anti-spam measure which deserves to be adopted. It is appealing to some ISPs more as a lock-in tool against their users more than anything else.

Please read this Frequently Given Answer as to why you should not implement SPF.