I'm moderately new at shmups but I like the balance DonPachi has. It's like this inbetween that's neither bullet hell or traditional shmup. It's not too overwhelming but still gets challenging and exciting and hectic enough to be enjoyable. I don't know of too many other games with that tradshmup/bullethell balance, maybe Battle Garrega but the scaling difficulty makes that one more overwhelming. Sorcer Striker might be a good example of something similar, that has a balance of bullet density sort of like DonPachi's.
Maybe being a bit new to the genre is part of it, I like a small dash of ego food to warm up before the game starts kicking my ass.
Plus I just really like the aesthetics. Visual design and sound design are really charming, and the narrator is great. The style and everything just screams 'classic arcade action' in a certain nostalgic way, but there's enough going on that it feels fresh and detailed. In this surface level 'flavor' regard I like it more than any other shmup I've tried so far, and as shallow as it might sound that really does go a very, very long way.
Just a feelgood game I can slide into whenever. I always have a good time with it.