I got different backpacks for different requirements:
For bulk and adaptability (groceries, bike washing kit, clipping on additional odd shaped objects) Downsides (too many loose straps slap and sting the thighs at high speed, too informal for work, not water proof):
or something similar:
http://www.mec.ca/AST/ShopMEC/Packs/Daypacks/TopLoad/PRD~5024-091/arcteryx-cierzo-25.jsp
For fair bulk but formality also (transforms into a shoulder slung or briefcase for meetings, also has decent laptop padding and restraints). Downsides (not waterproof, too nice to filth up on a rugged demanding excursion):
http://www.google.ca/imgres?imgurl=...&rls=com.microsoft:en-ca&tbm=isch&um=1&itbs=1
For Laptop protection, aero dynamics, and cool factor (also impresses clients despite sporty look, designed for laptops, extremely water resistent, protective, great laptop padding, minimal straps - very tidy) Downsides (limited to 15" laptop, price, limited storage size, in a severe downpour some water will penetrate zipper...never got my laptop wet though):
http://www.axioluggage.com/swift_carbon.html
For extreme waterproofing / actually for swimming!!! (100% waterproof, trust me I went snorkling with it) Downsides (single zipper / pocket, price, limited application, can be negated by a water proof cover for a regular back pack for riding):
http://www.mec.ca/AST/ShopMEC/Packs/WaterproofPacks/PRD~5024-396/mec-aegir-20-pack.jsp
For those days when I don't have pockets and wanna take a couple little items (if I am in my styling jacket, full leathers, or whatever):
http://www.mec.ca/AST/ShopMEC/Packs/ShoulderBags/PRD~4009-376/mec-bon-vivant-shoulder-bag.jsp
And of course, the "foldaway pack" to stuff into your bike trunk for when you didn't think to take a bag, did some shopping and now don't know how to take it home! Downsides (not robust quality as it is meant for light usage):
http://www.blacks.co.uk/Rucksacks/T...102510/packable-daysack.html?attribute=165072