I’ve ridden with several front panniers and rear panniers, and with a variety of bike trailers, but I haven’t seen a bag quite like the Arkel Tail Rider. This sleek-looking trunk bag has many handy features which are applicable for long day-rides, or commuting around town.
My favorite feature on the tail rider is the expansion capacity. It can expand it’s width to almost a foot across. The top has cinching straps that you can adjust depending on your load. This allows you to tighten your items while riding, and makes it comfortable to carry while off the bike so no matter what size your load, it still totes nicely, briefcase style. A shoulder strap is also available separately if you like carrying things sling style.
The expansion feature is convenient for taking a full size lunch container to-go, carrying books, and even bringing a 10.5″ laptop plus carrying case. Taking a small, unexpected jaunt to the grocery store can be quite rewarding as well with the ‘how much can I fit in the bag’ game. I have yet to lose, sometimes even picking up enough groceries to last 3 days or more.
The Tail Rider also offers lots of organizational space. On each inner side of the bag, there are 3 sectioned mesh pockets. I carry all of my necessity items in here like my patch kit, tool kit, utility knife, spoon and fork, ankle cuffs and other bike-related-just-in-case things. My pump and U-lock fit nicely in the large main compartment. It also has 2 outer side pockets (one on each side) for easy to get items like keys, wallet, and chap stick. The front of the bag secretly holds a bright-yellow rain cover which you can quickly throw on the bag while it’s still on the bike. This rain cover has a reflective stripe on it, and a loop for clipping on a light if it’s really ugly out. It also includes a removable central divider, but since I’m always filling this thing pretty full, I don’t really utilize this feature.
Some other features that are helpful, but not so obvious are the large, waterproof style zippers and the 4 tough Velcro straps which attach the Tail Rider to your rack. You can zip over large bulges with ease, and your bag is secured tightly to your rack. The bag opens up the center with 2 heavy duty zippers, and has a Velcro flap that secures the end. 
This lets you easily load from the top and close it up quickly. The handle is also squishy and comfortable to carry (say, if you had any recovering broken appendages to deal with).
All in all, I think the Arkel Tail Rider is a great bag, and fun to carry around on errands, or keeping it light on long rides.