I use subs to escort surface ships and watch out for other subs, and use them for pickets like everyone else. Other than this, regular subs are probably best in wolf packs. Subs are relatively cheap and quick to build. Once in a game I had the Iriquois attacking with 4 battleship escorted carriers, and what little navy I had was sunk by an Iriquois pack of like 10 subs! I figured if I built battleships and sent them after the carriers and battleships, they would get torn apart by the bombing, besides taking forever to build anyway. I put together my own wolf pack and eventually chased away the carriers! I like to have a nuke sub loaded with a tactical nuke sit off the coast of the other civs by their capitol or other HVT so I can whack them good if they get out of line.