My exposure to Mizuiro series is limited to the second OAV (non-adult anime), but I picked up four drama CDs at a used CD shop in Akihabara because I liked the OAV, like drama CDs in general, and wanted to see what Gotou Yuuko's acting was like in the old days. Although none of the drama CDs contain adult content, they are all related to the H game. You can tell because the voice actor credits use different names. In case you didn't know, Gotou Yuuko (Mikuru in Suzumiya Haruhi no Yuuutsu) is also Touno Ran. Oh wait. It's supposed to be a trade secret. Don't tell anyone.
The story is based on the overused theme of characters switching bodies. It's easy to do in drama CDs (and anime, too), but it gets tiresome really fast unless done extremely well. Unfortunately, the script isn't very well written, neither is the voice acting particularly good. But the songs are good and the omake talk is interesting enough, so it's not a total loss. Besides, I only paid Y480 for it, so it worked out fine for me.
For the drama portion, mastering is done well, but the recording itself has some clipped areas. I'm guessing low-end equipment or bad set up. It doesn't clip very often and even when it does, it's not very annoying, so I'm not taking away any points. Both vocal tracks, and particularly the embedded one in the last track, are mastered fairly well. All combined, I have no problem giving it 5 stars.
