Unless you have a drier or some kind of heater to dry out your wet clothes, getting soaked on the way in to work results in wet clothes going home, not fun and the recipe for chafing/blisters. Wet shoes are the worst IMO! Damp isn't so bad, that will dry out in a normal office, wet is the problem.

A rainshell that is fully waterproof (as mine seems to be after a downpour last week) also doesn't breathe so on a warm day, you'll be soaked from sweat. On light rainy days, I'll just wear my "baggy" windshell that is water resistant which will keep me essentially dry but it does allow a bit of airflow so I'm not so hot or sweaty. On cool to cold days, my waterproof rainshell will be fine because the cool/cold air will prevent me from overheating too much.

I will carry a second pair of shorts and socks on wet days but I'll wear booties to keep my shoes dry or just limit them to being just damp. I also have a helmet cover to keep my head a little drier which on cool to cold days, is appreciated.

On cold (near freezing), rainy days, I might not go out because, on cool to warm days, I don't worry about my legs but on really cold days, I currently don't have anything that will keep them dry.

Gloves, this is also an area that I haven't yet "solved" for cold days, for cool days, I'll just ride home in wet ones or bring a second pair. I haven't yet got them but you can get disposable latex gloves from your hardware store that, if you buy oversized, will fit over your cycling gloves and keep your hands dry and perhaps a little warmer too. A lot cheaper than proper waterproof cycling gloves (full fingered of course).

My 5¢ (no more pennies in Canada)