I loved my type IX B uboat, and used it from late 39 to early 43. When out on patrol my biggest hindrance was the lack of torpedoes. Since I play with the GWX mod, I use 2 torpedoes on any of the moderate sized ships or bigger to make sure they sink.
I was always able to sneak in between the front central escort and the side escort, in order to get among the convoy. I would fire off all my torpedoes, which at two per ship would be 3 kills of large ships. Then I immediately dive going silent and deep to sneak out. After I get away, I reload my torpedoes and bring in new torpedoes from the exterior storage bays, and then launch a new attack on the convoy.
I also loved the extra fuel, even though I mostly fought close to Great Britian, the extra fuel meant I could run the engines at a faster speed. Thus I could go head out and return to port much more quickly, allowing me to complete more patrols in a given year.
When I got into the IX D2, I was in heaven

Extra torpedoes and tons of fuel, meant I could move at flank speed all the time, and rampage around the Isles.
So yes, type IX's are awesome boats when you do hit and run style attacks.