We flew delta out of Buffalo for our last trip, I would rank them about the same as all the other major airlines (United, American, air canada, etc). Service was fine. DH and I both prefer either JetBlue or Southwest if driving to the states to fly is an option, or WestJet if leaving from TO, but if delta or one of the others is cheaper we go with those, the differences in service/seat space aren't enough to trump price for us.
One exception would be if you need to check a lot of luggage, then southwest really has an edge with their free bag policy