The bent arm pullover is one of the prime mass builder for the upper body. The straight arm version works well but not to the extrent that the bent arm version does because more weight and a better stretch can be achieved. Using DB's can reduce some undo stress on the wrist, elbows and shoulder girdle in most men rather than using a BB, though both styles work well.
Pullovers work the lat's, pec's & triceps very, very strongly but go easy on this exercise at first, if it's new to you. Even the abs, as a stablelizing group, are worked hard. Combining the bent arm pullover with the BP, as a single compound exercise (pullover & press), increases the capacity for muscle mass for the upper body. You can either do one rep of pullovers and than press the weight (either DB or BB) overhead right after, than do another pullover, press, pullover , etc. Or do reps of bent arm pullovers and immediately do overhead (bench press) after, with the same weight for one complete set. You can get a better stretch on the pullovers if the knee's are drawn up rather than have the legs out flat if your doing light pullovers after squats, etc. For heavier pullovers it's a pretty good idea to rap the legs around the bench or even have some one hold the legs down. You can, after a while, use large amounts of weight in this exercise. 150lb+ DB's or 250lb BB's are not out of the question
Rib Box: If young enought, the cartilage (elastic tissue around/between the rib's/breast bone) is still in a soften state and has not harden as it does when we mature, usually around 22 -24 years of age in most people. Pullovers will allow a expanding influence on the rib box to reach a permanent size increase in the chest measurement. A lot of younger guy's have had a 2 or 3 inch gain on the chest. Of course if you include a serious set of pullovers after breathing squats than the gain is almost assured in most people. Even in older trainee's there can be a noticeable flexability not only in the rib box but also the muscle attachment/inserts around the chest area giving a fuller, higher feeling to the chest. Good Luck.