From: Chicago, Illinois
I agree with Mr. Solli, and will expand further on IJN Subs, though I am not the expert:
1. I think the GLEN subs are too valuable for attacking ships. They are great scouts, and I use them as such. A line of them can provide early warning of invasion, or spot CVs.
2. The other classes can be attack subs. Allied ASW is pretty good, so attacking defended targets is often fatal to the sub. So is parking in a port. I have had most success in trailing a battle and looking for cripples; any damaged transport or ship falls quickly to an IJN sub. I have sunk a CA, CVE, DDs, and lots of transports that way.
3. I have had some success with a wolfpack between SF and PH; a couple GLENS to spot convoys, and some cruiser types to attack. Mostly I think it just forces the Allies to escort and convoy all over the map.
4. The IJN Minelayer subs, I-121 through I-124, should focus on laying mines.
5. For future builds, I prioritize subs this way: GLEN SUBS: You want them all. ATTACK SUBS, I-171 and UP: Yes to those RO BOATS: No on the short-legged, maybe on the others, though they only have 2 TT up front. Most are not that useful. TRANSPORTS: Yes, and accelerate. You need them in dark times.