One of the big things that seems to put off potential bike commuters (other than the ever-present question of, "How do I un-stink at work?") is how to schlep one's junk back and forth.
Having commuted for a long time, I think I've tried just about every combination of bike luggage out there. I'll save the specific brand/model reviews for a second post, but here's a breakdown of the general factors to consider when you choose your baggage.
BACK OR BIKE? This is the biggest dividing line in bike luggage. Do you carry your stuff somewhere on the bike itself, or do you carry it on your body? Bike-carriage has the advantages of reducing Sweaty Back Syndrome (SBS) and possible back strain issues. Disadvantages can include the need to mount specialized stuff (racks, baskets, saddlebags) on the commuting bike and bike compatibility issues (short chainstays plus large panniers don't always mix, for example). Making yourself the mule means that your baggage is immediately compatible with just about any bike. However, schlepping kit on your body also brings with it the above-mentioned SBS (the flipside advantage for winter commuting is extra insulation back there), and a poorly-placed or heavy load can't be good for your spine.
Within the bike-as-mule camp, you have several options -- a front basket, panniers or a trunk on a rear rack, or a big saddlebag. Front load can provide handling challenges if your bike isn't designed for it; I highly recommend Bicycle Quarterly's analyses of front-end geometry if you're picking a bike specifically to carry gear up front. Rear panniers are a bit more forgiving, though a light, flexible bike can get a "wag the dog" effect from a heavy rear load. I can't really speak to big saddlebags, as those are the one thing I've yet to try.
The body-carrying breaks down into a couple simple categories: Backpack or "messenger" bag. I scare-quote that second one since they've become so ubiquitous among people who don't even know what a messenger is, it's hard to connect them back to their origins with real working cyclists. I've used both types, and honestly, if you choose good ones, I think the differences come down to style. Messenger bags do provide easier access "on the fly" if you need to grab something out of your bag or shove something in there without taking it off. I'm using a backpack now, but -- I have to confess -- a large part of it is a perverse desire to not look like a fixed-gear hipster wannabe.
THE TRANSITION: This is an aspect that I'm not sure a lot of commuters consider until they're actually out there doing it. How long does it take you to get from "riding mode" to "locked up, bags in hand, on-foot mode"? I see a lot of riders struggling with hard-to-remove bags, multi-step pocket emptying, and awkward locking techniques. It's no wonder they don't like to commute. If you had to unstrap a NASCAR safety harness and crawl out the window of your car Dukes of Hazard-style every morning, you'd probably hate driving to work too. Bags-on-body wins here, since a big part of your transition is done as soon as your foot hits the ground. Still, smooth transitions are possible with bags-on-bike, either by carrying a body-mounted bag in a basket or by choosing a pannier with quick-release hardware. If you're an all-season commuter, consider how easy that hardware will be when you're in bulky gloves or mittens, too.
MOISTURE MANAGEMENT: There is just nothing worse than riding in the rain, getting to work, drying off, and finding out that your work undies are sopping wet in the bottom of your bag. When you're reading manufacturers' buzzwords, "waterproof" and "water-resistant" sure sound similar, don't they? They don't behave similarly in practice, though. Seams, hardware mounting points and zippers on water-resistant bags can all provide entry points for water, leading to damp undies. You really have to make a choice based on your local climate and commute distance, though -- for my short commute here in the Midwest, I can get away with less-sealed bags. Seattleites will tell you something different, though. A water-resistant bag can always be supplemented with a waterproof roll-top dry bag, too.
VISIBILITY: A bag that's big enough to carry a change of clothes and a lunch will by its very nature provide a decent amount of surface area. Use it to your advantage. Get a bag that has plenty of reflectorized accents or add your own. Choose a bright color that provides plenty of contrast against your surroundings. Mounting points for blinky lights are a nice add-on, but don't be lulled into a false sense of security by them. Most blinkies are extremely directional -- if a driver's the slightest bit off-axis to them, the light doesn't look nearly as bright. Since a light on a bag relies on the position of a floppy fabric loop and the position of the floppy fabric bag on your body or bike, the chances that you'll actually have the thing aimed where it belongs when it counts are pretty slim. Hard-mount the blinkers on your bike and use your luggage for reflectives.
SIZE: Bigger isn't always necessarily better. A bag with a lot of unused space will flop around or fit awkwardly on your body. Pull together whatever your standard commuting load will be (change of clothes? lunch? shoes? laptop? files?) and pick a bag that holds that with just a bit more room to spare. That will allow you to fit bulkier clothes in the winter, stop at the store on the way home for a couple forgotten groceries, or bring donuts for your coworkers (an important peace offering if you haven't figured out that "how to stay un-stinky" commuter issue). The proliferation of really compact reusable grocery bags can come in really handy -- stick one in the bottom of your bag and use that for those "special occasions" where you have load overflow rather than picking a giant bag that's underused 90% of the time.
Next up: Some brands and models of bags that I've actually put through their commuting paces.