I recently purchased it off steam, It was pita because I expected it to be the most up to date versions which it was not. I diddnt realize that so I had all kinds of problems trying to run mods, play multiplayer ect. I know it was stupid, but also easy to over look as you would expect steam to have a lateset version.
If anyone is thinking of getting steam this will make it a ton easier--
Steam gives ArmA 1.08 ---> patch 1.14 full ---> patch 1.15 beta (if desired)
Ace 1.05 full -----> Ace1.06 ----> ACIEP (if desired)