It's very useful for checking syntax, and pointing out exactly what parts of the message have triggered on a particular rule. Failing that the next step should be to test your installation using
Spamassassin --test-mode < sample.txt
Where sample.txt is a text transcript of your message. This will test SA's rule matching on the exact mailbox.
Actually, before you get as far as using --test mode.. make sure you run lint to look for typos in your config.. this is a VERY common problem:
This command: spamassassin --lint
Should run and exit quietly with no output.. if it complains, you need to fix your config files..