If you commute on your mtn bike, I'm sure all the folks who say "never" here would change their mind
I dislike backpacks in the summer between the sweat and the center of gravity and all that. I run an OMM Red Rocks on an old Marin Team steel hardtail with an old Marzocchi Z2 on the front. I have a front rack for it too when touring.
Post racks don't seem that sturdy to me. most of them do have 25lb limits on them. Carradice makes some fancy saddlebags that are more well supported than a post rack.
Jay