I too have recently made the bike commute commitment. 26-30 miles each way (depending on route) & a bus return trip. The weather difference between Seattle to the West and Bellevue to the East can be a torrential downpour and no clouds at all & combined with the weather difference between 15 miles North of either downtown to 15 miles South makes judging what the weather is, or could be problematic, at best.

If I was to make the commitment at all, then I felt a full kit was a must. I picked up Ortleib panniers and rain pants at REI, some fenders ($6 at Goodwill!) A Performance bike dayglow coat (on sale), some neoprene booties and some extra large goretex gloves at an LBS, & a cycling cap at a local co-op.

1 pannier has rain gear, the other a days worth of cloths.

The Performance bike jacket really seals well around the helmet, but doesn't really breath that well. Next time, I think a Goretex jacket might be worth the expense.

The impulse purchase neoprene booties are worth every penny, almost making my Lake 303's redundant. (I wish I'd discovered them sooner, I'd have cancelled the Amazon order for the shoes)

The extra large goretex gloves keep my hands toasty & dry inside my regular cycling gloves. I tried ski gloves and wet liner issues were just a PIA.

Least important: I find my leggings keep my legs warm whether they are wet or not, so I think rain pants are the least important unless it's raining hard enough for water to run down your leg into your shoes.

Most important: jacket with good hood that seals well against helmet and face. A good second would be a skull cap that keeps the wind and the added wind-chill of cold rain off your dome.

I have yet to be miserable like I imagined I would. In fact, with decent gear, it's downright pleasurable.