(I wanted to call it the ups and downs, but being in the Netherlands it would be more the flats and flats)
Pro: You get from A-B faster than walking (and public transport).
Con: I equal the time out by getting tangled in my locking mechanism.
Pro: It's a good way to warm up for hockey/football.
Con: It sucks to cycle home afterwards.
Pro: You can point and laugh at the suckers stuck in traffic as you sail past.
Con: The suckers stuck in traffic can point and laugh at you when it suddenly starts to rain.
Pro: It's safe. Bicycle paths and dedicated traffic lights mean you don't need to dodge cars.
Con: It still gets confusing as to which side of the road I should be on.
Pro: Drivers are assumed liable if a cyclist gets hit. No matter what the circumstances.
Con: Regardless of who'll be paying the medical fees, if you get hit by a car it's guaranteed that the vehicle won't be the one taken off in an ambulance.
Pro: Lots of fresh air gets the brain into gear on the way to work.
Con: Lots of fresh air usually comes in the form of very strong gusty wind. (Also, some bugger in a van insists on spritzing his windscreen wipers down the same stretch of road, at the same time every morning - resulting in a face full of watery cleaning agent for me. Fortunately I don't cycle with my mouth open any more. It used to be very unpleasant.)
Pro: I can cycle with my phone/mp3 player on.
Con: I still can't cycle while holding an umbrella.
Pro: My sixth/seventh hand bike is still considered 'new-ish'.
Con: Parts of it are held together (literally) by cellotape.
Um... that's all I can think of for now. I'm sure there'll be more. Actually haven't been able to do much cycling lately, what the lashing winds, heavy rain and wintry temperatures.