I live in Seattle, I ride year-round, and I swear by my Roadcrafter one-piece suit.
It works year-round in Seattle. What more do you need to know?
The guy who works in the office next to mine got the Aerostich two-piece. He now wishes he got the one piece. I can get my one piece on and I'm walking out the door helmet in hand while he's still trying to get his pants on. Plus, there's no gap for water or cold air to leak through in the one-piece.
I don't use heated gear, even in temps down into the 20s. The suit keeps all the cold air out, and my warm air in. This morning, it was 26 when I left the house, so I wore a heavy 400-weight fleece under the suit, and wore my wool-lined gloves. Combined with heated handgrips and handlebar covers, I'm toasty and warm for the 30-45 min. ride to work. If I were to spend the day riding in cold weather, I'd add a sock layer and wear long-johns under my jeans.
When it's hot, the Aerostich can get hot as well - until the bike starts moving. As long as the bike is moving, the back vent is unzipped, the sleeve cuffs are opened up, and the pit zips are opened, tons of air will move through the suit, keeping you amazingly cool even in triple-digit heat.
A few years ago, I was riding across the Cascades, from the high desert of Bend to Portland. We dropped down from the mountains into the Portland area on a 105 degree day. It was late afternoon, and the sun was blasting us in the face the whole ride west. The moment we had to stop for a stoplight, the sweating began - but the moment the bike started moving forward again, the vents started scooping and venting air, and the sweat evaporates in a few minutes.
After five years and 60,000 miles, it's showing it's age. I've had to replace the hook and loop fabric in a few places as it wears out over time. I've converted the collar tab from velcro to metal snaps. The giant chest pocket zipper failed about a year ago, but I can't bear to part with the suit to have it fixed. Someday I'll get a 2nd suit, and then I'll send the old one in for repairs and updating so I'll have the perfect back-up suit.
Definitely get the full suit instead of the two-piece. I love the small zippered pouch that velcroes to the left forearm. That's where I keep my key, my gas receipts, and my space pen to write mileage readings on my gas receipts. It's also great for change, toll tokens, a credit card and license, etc.
I wash my suit two times a year. I remove all the pads, and then wash it once in our front-loading machine using regular clothes detergent. This removes most of the oil, grease, and grime. Then I run it through once again, this time without soap, to remove all soap residue. Dirt adheres to soap residue, and there is no faster way to defeat Gore-Tex fabric than to wash it once with regular laundry soap.
Then I run it through yet again, this time using Nikwax "Tech Wash". This shampoo is designed for Gore-Tex items, and removes much of the dirt, oil and gunk from the pores within the Gore-Tex fabric, restoring breathe-ability and waterproof-ness.
Then I'll run it one final time, this time with Nikwax "TX Direct", which is somewhat like Scotch-Guard in that it restores the water repellency to the outer layer of fabric, helping water bead-off before it can soak in to the fabric.
I love my suit, and consider it my seatbelt-airbag-crumple zone-life preserver. When people ask me how much it costs, my stock reply is "less than a skin graft".
The best anecdotal story I have regarding Aerostich suits:
My next-door-neighbor is the Sgt. in charge of the neighboring county sheriff department's motorcycle detachment. He periodically leads track days for motor officers with his agency and for other area law enforcement agencies.
A few years ago, at one of these track days, a motor officer was riding around the track at high speed on his R1150RTP. He went a little wide around a corner, the bike went off the pavement, and immediately dug into the soaking wet grass and earth. The front tire stopped immediately, and the bike - and officer - did approximately 8 or 9 high-speed somersaults down the track.
The life-flight helicopter came and airlifted the deputy to the trauma center. The ER doctor cut away the suit, and proclaimed that the suit had likely kept the deputy's arms and legs attached to his body throughout the horrifically violent crash as centrifugal forces tried to dismember the rider. He had broken bones, but he was still alive and intact.
That's all I needed to hear.
I know I'm not invincible in it, but I know I'm much safer wearing it.