Switched from full rack/panniers to seapost rack & trunk bag
When I started bike commuting a few years ago, I had lockers and a shower at work, so I'd stock up the locker once per week and only carry a camelbak. About a year ago, I got a new job and shower about 2mi from my job, but I have to carry my stuff. I've always shared my commuting between an '80s Schwinn (currently fixed gear) and a triathlon bike. Immediately, I added a rack and some panniers (Nashbar Euro-style) to the Schwinn. When I'd ride the tri-bike, I'd use a mini-u lock & pump mounted to the frame, a typical seatbag with tools/tubes and I'd put my clothes (I can wear shorts at work) in zip-lock bags and stuff in my jersey pockets along with my cell phone, keys, and ID & credit cards. I've also done the backpack thing, but it slides up too much when in the aero bars.
This worked, but I didn't like the handling of the Schwinn and I really don't like my jersey <i>that</i> weighted down. So, I decided to try out the Topeak seatpost beam rack with their small (Rx line) trunk bag that I could use with both bikes. Today was my first ride with it (on the Schwinn) and I love it. The size is small enough so I had to streamline what I bring, but still big enough to carry a change of clothes, mini-u lock, cell phone, pump mounted underneath on the beam, 2 tubes & tools, a hand-towel, and it has some fold-down "panniers", ~ 8"x10"x2", in case I need to carry a little more.
Because I had the space, I've always carried way too much in my panniers (big u lock + cable, 4 tubes, more tools, etc). I'd bet I had 5-8lbs more in that setup than I'm at now. I know I could just ditch the full-sized panniers and run this trunk bag on my bolted rack, but I also really like the slide on compatibility of this combo and I have things mounted to the rack (blinkies, pump) that is needed for both bikes.
I've posted this because I've read alot of seapost rack bashing here and to say how much I like the Topeak setup.
I've been thinking about doing this, but the fact of the matter is I haul more stuff: change of clothes, toiletries, bike stuff bag (tube, pump, levers, allen wrench tool, light rainjacket), wallet, cell, breakfast + lunch, and often a 1/2-inch to 1-inch stack of work (I'm an attorney with a newborn and take work home often). I would need a larger trunk bag (something that holds 1,300cm2+)