It was a pretty simple process. Deceptively so, for $150. Two guys came with a big shop-vac and a big brush (the diameter of the chimney) with lots of attachable handles.
They scrubbed up the length of the chimney and vacuumed up the debris. Not that much came down. I imagine we could have gone another year.
Afterwards one of them walked on our roof. Which I wasn't pleased with, since we have a slate roof and the slates are rather brittle. Hopefully he didn't crack any of the tiles. He then took a few terrible cell-phone photos of the inside of our chimney that exhausts our oil-burning furnace and informed me that the liner was totally degraded and needed to be re-lined immediately. For about $2000. Hmm
From some Google-searching it seems that chimney cleaners are always recommending new liners. Our porcelain liner is about 55 years old. They can last up to 75 years. Maybe ours does need to be replaced now - but I'm not inclined to spend $2000 based on word-of-mouth and some cruddy cellphone pictures.