My high school Physics/Chemistry teacher was a captain in the US Navy and served as a submarine skipper during WWI and WWII. After WWII he took a type XXI U-Boat for a shake down and subsequent refit for service in the US Navy. He loved the type XXI boat.
All the non-nuclear submarines post-WW II are the grandchildren of the Type XXI.
After WWII, the Type XXI was pretty much the basis for all new sub construction until the USS Albacore, AGSS-569, which became the basis for current submarine design, starting with the Barbel and Skipjack classes.
http://en.wikipedia.org/wiki/USS_Albacore_%28AGSS-569%29