I find it hilarious how the best episode is literally a hentai episode. For once, they got rid of all that generic demon plot bullshit mixed with unnecessary drama and focused on the slice-of-life aspect the series attempted to have. Because of that, this episode was much more enjoyable than I had anticipated. For once, there was consensual sexual tension instead of being enslaved by jealousy or the idea of betrayal. This OVA also focused on Maria, which is good because she's the show's lifeblood; without her, this anime would fall into obscurity. Lastly, if you're going to watch this series, I'd suggest watching this episodeas it's the least cringe-worthy of them all which is saying A LOT.
Sadly, you'll have to watch the previous series for context purposes. Good luck with that.