Yes, it's hard. Yes, the learning curve is somewhat Everest-like. And it will reward every minute you put into playing it, learning about it, thinking about it. It's that deep. The several patches (3.26 was released this year) have not only fixed (a few) bugs, but also added new content. It also comes with Windows, Mac and Linux versions on the same disc, or it can be downloaded.
My previous all-time TBS game was HOMM 3. This game makes my old favorite look like checkers.