First, let me say this is NOT a first person shooter. This is a detailed, "all the widgets are under your control" submarine simulation.
The learning curve on this game can be prohibitively steep but there are some great tutorials and let's play videos on you-tube that can get you up and running.
The game is actively supported by the folks over at subsim.com and there are more MODS than you'll ever need.
The game can be manageable but enjoyable if you go for auto-targeting but the manual targeting option is a bit daunting. I wish that it were more granular. For instance, I could see separating the task of ship recognition from the trigonometry of torpedo aiming.
One word of warning... even at its most difficult setting (called 100% realism in the game) it's NOT really challenging enough for many serious submariners. There are a number of mods that make it tougher.