I don't like milk crates because they crowd up against my butt when seated, and are also hard to swing my leg over on mounting/dismounting. Panniers also hold the weight of your cargo lower than a milk crate, which improves bike handling/stability. However if you get panniers check for heel room before you buy. If they do seem to crowd your heels as you pedal, it may be possible to position them farther back on the rack, or sometime the rack itself can be raised by screwing out extenders where the struts attach to the rear axles, and that can also help. Or else you may be able to lengthen the connectors that attach the front of the rack to the bike.

Having said that I sometimes do bungee or tie a crate to the rack and that is not difficult.

A back pack can be bungeed to the rack. I also see a lot of people carrying backpacks or other bulky items in a fold-out/collapsible wire basket that attaches to the side of the rack.