The fast track is a great option for people planning on going into academics/research, and who know early which specialty they want to do. You pretty much have to know before you start residency, because you apply to fellowship programs during internship. For GI, fast trackers would have to apply around August/September of their intern year to get a spot that starts after second year. They pretty much know which program they'll be going to by November/December of that intern year. For other specialties, they can wait until around November of the intern year to start applying, and then will secure a program around the spring.
Most people I know who have fast tracked have started off as clinical fellows right away after their 2nd year, and do the research later, so that their research time can be lumped all together, but I'm sure there's variation between programs. Normally in academic IM subspecialty fellowships, fellows do 1-2 years of research after their clinical training is completed, depending on the subspecialty. Fast trackers get an extra year of research time added on to this. So fast tracking doesn't save you time overall, but you take more research time so that your clinical training is shorter.