This was one of the first shoujo-ai animes I watched, so it is kinda special for me. It starts as a gender-bender and leads into a shoujo-ai anime, mixing aliens, romance and yes, like many have mentioned, a dose of angst and also of comedy. The animation and vibe is pure 2000's. You have versatile Ueda Kana onboard as the protagonist. She was very cute in this one, then badass in the Fate franchise as Toosaka Rin. How does she do it? I don't know. But it is truly awesome.
Anyway, back to Kashimashi. There is a little detail, that others have already mentioned, and I will mention it again. To be creative, Episode 13 is like the OVAs for Kimi ga Nozomu Eien: it represents an alternative route which may be pretty venomous if you liked the series. So, if you want to avoid frustration (and heartache if you are passionate), you know what you have to do. Don't say the other recommenders and I didn't warn you...
Overall, a good 2000's shoujo ai anime. Highly recommended.