I was a big fan of it, because it meant in favorable conditions I could hoard torpedoes. Nothing worse than sailing on your way back, finding a vulnerable convoy and only having 1-2 eels left. I've also had a few patrols where I ran out of torpedoes, but had enough shells to sink one last ship.
From 39-42 it is good. After 1942 even merchants start to get decently armed and using the deck gun (particularly in good visibility) becomes risky since you start to trade damage instead of simply inflicting it. some people have said you can use the gun to knock off enemy guns, but that hasn't happened to me (all the surface damage I've seen is cosmetic- crates on fire, cranes collapsing, etc). Never blown off enemy guns with my gun.
On my type VIIC I traded in the deck gun for a U-flak configuration. Surface action against ships has now become a foregone conclusion, though I'd love to hear of ballsy Kaleuns engaging in surface gun battles in 1945
