Well here is waht i like and dislike about the S-class.
+Smaller than the other subs = harder to hit
+Dives faster
+Since i installed Patrol mod v1.2f it does 11-12 knots submerged
+No stern tubes (makes it more exiting when hunted by DDs)
+only 4 torp.tubes and a samll amount of torpedos (makes it more challenging when attacking a convoy)
-It isn't the gratest looking sub
-Slow on the surface
-No stern tubes( sometimes i wish it would have stern tubes when a DD is right behind me coming straight at me)
-Only 4 torp.tubes (sometimes tit would be good with more tubes)
-Since i installed Patrol mod v1.2f it does 11-12 knots submerged
Note that some things are good and bad at the same time depending on the situation.
Final verdict: The S-class is good but not allways the best sub. It's best when in shallow water and in places where there is not too much room to turn etc.