Grapefruit juice and grapefruit became very popular in the 1970's thanks to the wrongly named "Mayo Clinic Grapefruit Diet" There is some research by Dr Ken Fujioka that shows the secret to grapefruit and weight loss is that it reduces insulin levels.
Not here to shoot down ideas guys, but I want you to consider something. Grapefruit is a simple sugar and stores as fat when not fully burned. Whne consumed in large ammounts this will continually spike your insulin and give you many unecessary carbs. I always tell me clients that fruit and dairy if to be consumed at all should be consumed in the morning so you can have the rest of the day to burn them off. Of course every case is different but if weight loss is a goal of yours consider grapefruit once a day for breakfast and be sure you have a balanced breakfast as well with plenty of slow burning carbs such as oatmeal and high protein such as egg whites or lean meats.
Kickstart your metabolism...most definetly, but tahts a response to the insulin spike and if eaten by itself theres a potential to store it more as fat and to make you crash depending ont he person. Just some quick thoughts. Loosing weight is not going be in some miracle fruit, when it comes down to it calories in versus calories out.