I personally love pref bidding. As an FO I was quickly able to get the trips I wanted with the days off I wanted. As a junior Capt I'm able to get the days off I want and maybe a trip or two I ask for. June's schedule gave me exactly the days off I wanted plus some and ClassBid was very creative in providing me a productive schedule in the remaining days.
The double edge of the sword is that it will often give you exactly what you ask for, and you may not have realized what you asked for was not what you wanted until the lines come out. In other words, it takes an investment of time and energy to construct a good bid that will get you what you REALLY want. The learning curve can be very steep and you may have to bite the bullet with some ugly schedules if you screwed up your bid.
Given our commutes to JFK, many of us at JB like to bid an average 5-days on, 5-days off. That means only 3 round-trip commutes per month. Other companies that construct 7-day sequences don't give you that kind of flexibility. Some guys even try to build 2-weeks on, 2-weeks off because their commutes are VERY UGLY. That is only possible by bidding for trips with duty rigs and accepting some down time on the road and/or back at JFK between trips.
Bottom line is that pref bidding can give you some great schedules with some very productive trips. However, you have to know what you are doing if you want to use it to its fullest advantage.