The Topeak system comes in at least 2 basic sizes (I believe the EX and MX series). The EX rack is intended for road bikes, or at least smaller lighter platforms where less capacity is needed. The MX series starts with a larger heavier rack and the larger bags (with or without the auxillary struts and folding paniers.) I have both racks and bags for both (without paniers or fold down pockets). Both series of bags fit on either rack. The MX rack is more beafy and comes with a QR clamp that fits a wider range of seatpost sizes. My EX rack must be attached to my 27.2mm seatpost with an allen bolt clamp. The smaller EX bag has approximately the volumn to hold something about the size of a sixpack in the main compartment with side pouches for tools and supplies such a tubes.
The principal advantage that I have found for day tripping with the system is that the bags also come with a shoulder strap. If it is necessary to lock up my bike where I might not have a direct view of it, I can hit the button on the bags quick release and take it with me, thus protecting camera's etc. (pop your computer off and slip it into the bag also). Topeak has an excellent online catalog to browse, a well deserved customer service reputation and pretty common availability through most LBS.
Edit: remember that either system will always have a top weight limit because of the seatpost mount and that neither is particularly suitable for carbon seatposts.
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