[Exim] Is SPF for me?

OK - Trying to grasp the SPF concept.

I do email hosting for a variety of domains, but I'm not an ISP. My
users are unsophisticated and spread out - often mobile.

Some tunnel back to my server to send email from "the source" but most
use their local ISP for outgoing SMTP service.

So - is this SPF for me? Can I use this to "bless" those who originate
from specific IPs without penalizing those who roam?

What would seem to make more sense to me for someone in my circumstances
is if the users outgoing email were relayed through my server instead of
to the destination. Then I could eval the message to see if it's forged
- perhaps requiring some encrypted password or key or something.

Anyhow - back to the original question - is SPF for me?