Trans-Island ChinaLink ($130) #Free transfer from Huanggang
China Travel Service and Eternal East-- #Same bus from Huanggang.
Myself prefer transfer mode--Cheaper AND more flexible.
(Same bus )If your coach mates had problems [something wrong in China immgration], The coach wait for these suckers for xxx mins.
(Free transfer)Just take your luggage and board another bus at Huanggang. That bus leave on schedule.
Airport Mainland Coach Services:http://www.hongkongairport.com/eng/cbt/cross.html
Have a nice trip!