I very much enjoy Dirge. As we all acknowledge, it's not winning any awards - except most underrated. But, as someone who does not frequent the shooter genre, I find the arcade-style shooting gameplay to be rather addicting. While obviously not on Shad's level, I usually pop it in and play through every once in awhile. The short length and being on a real console lends itself to being my most played of the Compilation save VII. I love nearly any opportunity to spend time in that world, and I always remember my first feelings of giddiness when, after the first mission, Reeve is talking about Heidegger, Scarlet, and Rufus. I love talking to everyone on the Highwind as well. Cid's dialogue is great and the small bit of word-building you get from the NPCs is appreciated.
I also like the soundtrack, and Jason did me the favor of posting my favorite track. Silent Edge is so atmospheric, I love it. But I do wish Hamauzu weren't so afraid of recurring series motifs (see also: XIII). How did they make a Vincent game without even alluding to Vincent's theme?
That said, I've never done too much extra stuff. I've gotten the Guard N Barrel and beaten Hard Mode. But I've only ever gotten most of the Memory Capsules, and I gave up on the Extra Missions at Cait vs the Crimson. I always want to do them, but the memory of that mission ends it for me. I think if they had actually included the Multiplayer Mode storyline in them I'd have had more motivation to get through them. (I definitely didn't know about your loadout being based on your last final boss fight either.)
And yeah, as stated about the Ultima Weapon was always so expensive I could never work out how you could ever afford it in time to actually use it.