I really think this one has a good mix of different aspects of the previous games. Four characters and a shot type per each like in IN and MS, using focus for a different shot type, Youmu's charge attack, and collecting items for 1-ups directly dropped from fallen enemies and after spell cards for bosses instead of risking life and limb for a single UFO (oh the nightmares that gave me at times).
Yuyuko just showing up for kicks simply made my day. It's just her style to be around for a little bit of playful silliness, too. "Hi! I'm bored! Let's have some fun!" Kogasa's power to surprise people has reached meta levels twice now, and I don't mind her showing up. I'm far from the biggest Kogasa fan out there, but she's too durn cute for me to be upset.
I am leaning towards Yoshika as my favorite between her and Kyouko thus far. However, this is almost exclusively based on powers and appearance right now. Kyouko looks rather plain, Yoshika looks downright ridiculous. Being a bit scatterbrained (does she even have them left? Oh yuck bad thought) always amuses me too. Still, the characters are far from fully fleshed out in both canon and especially fanon yet.
I just don't have anything negative to say thus far. Glitches are there and I could argue that one of Kyoko's spell cards is a little too easy, but that's readily alleviated for a final version.
Stage 4 boss gimick: Reimu and Sanae face Myouren. Marisa and Youmu face Youki.
:V
Considering how they're both pretty awesome, I approve.
OH MY GAWD, LIEK, SOLID PROOF THAT MIMA'S IN 10D!
From the wiki: Yuyuko - 何か、厄介な奴が復活しそうとか - I wager some troublesome individual will make a return.