advantages - you can take it with you when you lock your bike up, no weight directly on the frame.
disadvantages - sweaty back, back aches.
advantages - haul lots of stuff, cheap
disadvantages - not water proof
advantages- no weight on frame, easily detached, haul lots of stuff
disadvantages - expensive, extra tires to worry about flats, heavier
advantages - waterproof, lighter, easy to make a DIY version
disadvantages- weight directly on frame, worse handling under load.