I'd rather not play the guessing game, but I assume you just don't like it but you like treasure's other games? I do prefer sin & punishment: star successor and alien soldier to rsg and ikaruga.
Heh, it's not that I don't like it, it's more like I don't get all the praise it gets (same case with Ikaruga and RSG). Of all the Cave shooters I've played so far, I'd rank it somewhere in the middle. I picked up the port on release since I bought into all the hype from places like the shmups forum and n-g.com's shmup community, and I ended up being somewhat disappointed. I'd probably think better of it if I hadn't bought Futari 1.5 before it, since I think Futari 1.5/BL is superior in pretty much every category (scoring, visuals, music, difficulty curve). The Ketsui port's arrange mode and AST are much better than the arcade version, but it's still not as good as Futari. Because of that (and now SDOJ), Ketsui ends up feeling like a redundant piece of my collection, so I feel I'd be better off with something else in its place.
And since you brought up Futari and Deathsmiles being popular, I want to mention that I completely agree with you there. But I'd go a step further and say that they both deserve all of the popularity they've received. Particularly Deathsmiles, which is Cave's best shooter in my eyes (though to be fair, some of its best features were shamelessly stolen from the original Otomedius).