I had picked up some Castelli bib shorts from Sierra Trading Post last year when they had them on closeout (50% off - yay!). I find they are great for short bursts and wear them on the weekly shop ride, where I know p-breaks are not an issue. They are very comfortable, provide good coverage for shorter jerseys, and I've never had any problem with rubbing, chafing, or falling straps. Sometimes I wear an underlayer, sometimes not.
Are they better than regular shorts? Not necessarily. I wouldn't keep only bibs in my closet, but they have their place.
R