Silent Hunter 4 is a serious sim. You will have to devote lots, lots of time if you decide to play it. I bought it off steam while on sale, have over 7+ hours logged and have only sunk one ship outside of the tutorial. Some missions just have you scouting, no aggressive maneuvers. I wasn't able to keep on board, so i abandoned ship, paddled my dingy raft to land. Maybe I just didn't "get it"
Although having bought both, I've only played the expansion. For me was not buggy at all and from what I hear, have experienced nothing contradictory, improves the game significantly. (vista 32)
It is a very impressive looking game and would suggest buying it for that, but if you haven't the time spend your money elsewhere.