Crap; all seatpost racks are crap. Get a real rack and you will be able to use real panniers or make your own out of 5gal buckets. What kind of bike do you have? If it's full suspension that may be a problem.
Take a look at the topeak racks and bags (topeak.com). I use the MTX seat post rack with an MTX bag that slides on the rack via an integrated system. No bungies needed. They also make pannier side attachments that install in grooves on the side of the rack. Their bags use these sides to hold zip-out panniers. Stuff works great. It's limited in weight it can support (25 pounds?) but good for commuting. They also make a fender that installs under the rack. Very clever and sturdy engineering.