When I first bought the game the only upset I had was the AI. Coming from SH4 with TMO I'm used to needing an extra change of underwear when I attack convoys. With this game I could sail into Scapa (I literally did this 3 times..) and get all the tonnage I want and then slink away..
This AI feeling, plus the having to start a campaign always from 39 and not from say 41 or 42 let me to just play on and off. However, after downloading Feared Hunters and Uboat Killer AI Mod, the AI is back up to par. Merchies even evade after one has been hit!
You can really see the potential in this with Mods attached. As for bugs I haven't really seen any other then the crew on deck (when submerged) and kamikaze merchies in Kiel. DRM for me is a non issue as my firewall and Norton are up to date and my connection is a good one.
All in all, I've found myself playing it more and more with Mods. Excellent game and I look forward to its latest Mods and Patches