Depending on if I have another lock / shadow priest or mages in the group and if its trash or boss depends highly on what dots I use. On bosses I tend to use CoA, corruption, and Immo then shadow bolt spam. If there are 2 mages I'll nerf my dps by putting up a CoE.
My demo lock is currently at +1258 shadow dmg. Shadow bolts hit bosses on average of 2000+. I dont know, maybe most of the locks I have ever run with dont know how to play but I don't usualy get out dps'd in boss fights at all. I've played all the trees but demo is the one for me...and I do alot of dmg. You just gotta be good with pet control.