Nagato Yuki-chan no Shoushitsu: Owarenai Natsuyasumi

Honestly, I wish they would come up with original ideas for once.

This OVA takes place during the summer break and we basically see them having fun together, while Kyon is struggling to find time to do his homework, exactly like in the infamous Endless Eight arc...

Thanks to Sugita's hilarious acting and Koizumi's weirdness I wasn't too bored througthout the 24 minutes of this OVA but that's it.

This is watchable, not even remotely enjoyable, like the whole Nagato Yuki spin-off.
I'm not a fan of the new character design either and I don't like the OP or ED songs. Call it nostalgia or whatever ... but it was way better when Aya Hirano sung them, instead of Minori Chihara (her singing voice sounds like a duck), and much more interesting when Haruhi was the heroine.

Be warned, it will bring back Endless Eight flashbacks - but in a good way. ;)
If you survived the endless eight then this is a fun reworking of that story. It has a much bigger focus on the homework session which had some fun sequences - like the card game. Overall, if you have a soft spot for the series then this is a big warm hug.
It doesn't add much to the series other than showing how comfortable the characters are with each other and they all get their own screen time with Haruhi, Kyon's sister and Asakura taking the lion's share. But with the ... story being 'Endless Eight' it doesn't give you any new plot.