If you currently carry a fairly large load on your bike, either using panniers or a large backpack / messenger bag, or if you’re planning for a bike trip that requires you to carry lots of stuff, you should seriously consider a bike trailer such as the Voyager trailer pictured here as an alternative. The more you carry in panniers or a backpack the higher your centre of gravity rises and the more awkward your bike handles. A bike trailer eliminates this issue entirely by carrying your gear lower than your current center of gravity. What’s more, it’s noticeably more aerodynamic that panniers and especially on hot days it beats the heck out of carrying stuff on your back.
Of course you can also carry more stuff in a trailer than with any other cargo option, so the more stuff you need to take with you the more sense it makes to use trailer. For anyone who hasn’t used a single wheeled trailer before it’s worth pointing out that it hardly feels like you’re towing anything at all. The trailer tracks extremely well following the natural line of your bike even in sharp corners, and it fits through pretty much anything your bike fits through. It’s also very easy to attach and detach from your bike, leaving almost no hardware behind to way down your bike when you’re not using the trailer. All in all it’s a great way to haul around your gear, and for $428 it will pay for itself many times over vs. using other forms of transportation to get your stuff from A to B.