Once logged into webmail, there's an options link at the top. Then you click on SpamAssassin configuration.
You can add whitelisting there.
As for how well it catches stuff, I have my Tag and Quarantine levels both set to 2. That may be a bit overzealous for some, but I'm pretty happy with it. Suspected spam goes to the Junk box anyhow, so I check it from time to time.