The downsides: depending upon the weight, the bag has a tendency to sway, but I've learned to pack accordingly, stuffing heavier items towards the rack. It should go with out saying that any pannier constructed from multiple materials is not waterproof. I throw a couple plastic grocery bags inside for those days with a chance of showers.
handle bar bag for years. In its place I often contemplate zip tying a wire basket to the front rack - I love the look of baskets - but the functionality of this double fabric canvas bag with strong leather handles regularly holds a tiny tool bag, a mini pump, plus whatever else I can fit inside. It conveniently holds a gallon of milk or paint can, all supported upon the rack. This bag's a keeper!
messenger bag has become my purse. I store the usual items plus a camera. Because it is secured to rack with velcro-type attachment, it is not designed to carry heavy stuff. This is a perfect bag and easy to sling over my shoulder when I enter the office.
I'm pleased with the overall functionality of all my upcycled bike bags and wouldn't consider returning to traditional panniers for short commutes.
What unconventional storage systems have you explored?