I played this game a lot last summer when I bought it on sale, and recently cranked it up again for a few games. I've only played as the Union versus rebel AI, as it is more fun to play on the offensive when playing solo. I find the Confederate AI competent and a tough nut to crack, but by the same token unrealistically conservative - I have gone several games where the CSA did not launch a single attack!
Part of the problem is the game design. It's area movement, and the "hexes" are too large to enable maneuver offensives like Lee's invasions of Maryland and Pennsylvania, or Rosencrans' Tullahoma campaign that ended at Chickamauga. Amphibious or riverine ops are the best way to outflank an opponent - the amphibious rules are well done. But for the most part both sides end up entrenching and the game devolves into a western front-styled attritional slugfest.
Also, units can't really attack (or move much during winter) unless leaders gain "initiative" which is difficult for poorly-rated leaders (and there's no shortage of those, particularly in the US Army). And the spotting rules encourage unrealistic shuffling of troops and leaders - the AI is pretty good at this, but I think it's a borderline "cheat."
I may sound like I don't like the game, but I do. With some tweaks to the map, and modifying the way spotting and initiative are handled (i.e. leaders without initiative may still attack, but at reduced effectiveness), the game would be better. I'm disappointed a WBTS II hasn't come out and doesn't appear to be in the works. There's a great game in here, but the current iteration just misses the mark for me.