Dave, I think you're talking about Ichitaro from Just Systems. It is the Japanese-language word processing software that competes against Microsoft Word. Yes, it has lost a lot of market share today but still is the diehard favorite in government offices.
ATOK is the "enhanced" commercial version of Japanese IME, boasting greater accuracy in kanji conversion, Japanese grammar check and direct link to JJ and JE dictionaries it comes packaged with. It is a great aid for writers in the Japanese language.
I am an ATOK user and like it very much.
However, I doubt its usefulness to beginner- or intermediate-level Japanese language learners. So, it's really up to you to decide.
HTH!