I have a bicycle crate in my rear rack (40L from memory). I can just throw my backpack and/or shopping in there and be on my way. No issues transporting when empty. I avoid riding in the rain but I guess a waterproof bag would help for that. It's durable, the main concern is the rear rack. I had to replace the cheaper rack that I bought last year after the welding snapped in a few places over time (I had it held together with duct tape for a while). My new rack should be much more sturdy this time around.
I have access to borrow a car which I do every few weeks so I don't need to over engineer my bike setup too much.