These things are really popular. Like Beanie-Babies-Happy-Meal-toys popular. Similar promotions in Singapore and Taiwan have resulted in long lines and rapid sell-outs. A quick search on eBay shows a large secondary market for them.
Here's what you'll find in the Hello Kitty Circus of Life line of plush dolls:
First, there are four standard dolls that can be had for $25 HK (~$3.22 US) with any food purchase. They're the Chimp, Clown, Animal Tamer, and Trapeze dolls.
There's one "special edition" doll, the Dare Devil, that you can only get by spending $70 HK (~$9.02 US) or more on McDonald's McDelivery 24-hour delivery service and paying $25 HK for the doll itself.
There's another "special edition" doll, the Magician, that you can only get during "Mac Time," which is McDonald's special evening menu that's only available from 6pm to 4am. It also costs $25 HK with any food purchase.
Finally, there's a collector's kit for $238 HK (~$30.68 US) that includes various Hello Kitty collectibles including Jumbo, a Hello Kitty plush doll in an elephant costume.