UFO is merely okay in my opinion, definitely not awesome-tastic. The UFO gimmick is annoying, even on easy mode, does not really work too well in a fast paced danmaku game. The music... I like Touhou's music from the very start up to MoF, and SA's music eventually grew on me. I find UFO's music however, to be rather sub-standard. The only songs I really like are on stages 1, 2 and 6, the rest I'm finding rather forgettable, even after multiple listens, and I can't see this music growing on me like SA's did.
Stage 5 is horrible. Dying to shit I can't see as the bullets blend in with the cruddy background. The only new characters I like are Nazrin, Kogasa and Byakuren. Murasa is alright I guess (she should really thank her memetic badassness), but Ichirin, Shou and Nue do absolutely nothing for me despite the fact that Nue comes with zettai ryouiki and Shou comes pre-shibari'd.
I really hate the battles with Ichirin and Shou as well, over-use of toothpick lasers that my eyes simply can't track properly, and in general, more nerve-racking than they really need to be. I'd rather fight Parsee, and I hate her battle with a passion. The other bosses are far more entertaining, but even they have their argh moments (the finals for Kogasa and Murasa, along with Byakuren's terribad yellow worm laser attack).
So for the second game in a row, I'm in the minority. I find UFO to be average at best, with below average music and underwhelming character designs (in the cases of Murasa, Nue, and Shou that is).