My DH did all ours this year. It wasn't all that bad of a job....just messy.
He used a spray bottle to wet the ceiling as he went, and used a large putty knife (6 inches wide) to scape with. He attached a garbage bag to a coat hanger and then scraped the ceiling into the garbage bag. That way very little went around the room. Be sure to wear protective eye and mouth/nose coverings.
We found that our ceilings were in really good shape and didn't need a lot of work after scraping them. The only thing is that you will end up with the drywall tape between teh ceiling and floors being loose. We decided to put crown moulding up on all our walls to cover that problem. He did that too!
If the house is older than 1980s then they should get the popcorn tested for asbestos before doing anything. It used to be an ingredient in the stuff. It cost us $25 to get ours tested.
Let me know if you have any questions.