With Princess cruises you can normally save some considerable dollars by researching and booking with an online agency.
That said... you don't have to actually book it online. Call or email agencies to inquire about their best price!
With some communication you'll get a better feeling as to the service the agency will provide. Once you find one you feel comfortable with, come back and post asking for some other folks personal experiences with that particular agency.
Just look for responses specific to the agency you're requesting info on, and take "with a grain of salt" those who respond saying use my agent, they are always the best. Those who answer a question about one agency, trying to direct you to another agency are always a bit suspect to me.