Personally, I prefer doing my own thing. If I have 60 minutes, I'll do the 60 minute workout. If I've got 45, then I mix and match a workout or two OR just do the 45 minute workout.
Nowadays I have 3 PE classes in college to attend to, so I'm physically active with that, but sometimes it's nice to be able to come home and just do a 15-30 minute interval workout on days I don't have classes. Honestly, the only times I felt like I wasn't getting results was when I was eating like crap.