Got the Topeak MTX Trunk Bag EX with the expandable panniers weeks ago, and today finally put it on. To use this bag, you need the rack that goes with it. Not a problem in my case since I didn't already have one. Looked simple to do, but it was a bugger to get the rack on. Very roomy, but compact. It slides off the rack, if desired, and has a shoulder strap to lug it around if you lock up your bike and have valuables you want to keep with you. The test will be quick trips to the store and farmers market! I'll give a full report when I know if it works as well as it promises.