The "To LOVE-Ru Darkness OVA" episodes follow the same formula as the Motto season, meaning that each OVA is split into a few "Mini Episodes", the biggest difference being that these episodes are not related to the main story at all. The episodes are however related to chapters from the manga, but from what I can tell (not having read the manga) they don't affect the main story in the slightest. This means that not having watched this won't make a difference, other than perhaps the "Prologue" episode, which covers a bit from before the Darkness season. It really involves our main cast getting into a lot of weird, funny and awkward moments without any plot or real character development.
The art itself is of course the same as the actual series, so in overall pretty good, and it being an OVA, you won't be pestered by steam clouds and whatnot. The same goes for the music, the soundtrack and sounds overall are all pretty much from the main show. The only difference is that it focuses more on the ecchi parts, but what do you expect from a side OVA.
In conclusion, if you're looking for a bit extra after having watched the anime itself, it's quite the amount of fun. However, don't watch this until you've watched the Darkness season, it will spoil it for you if you haven't yet. The characters are still enjoyable, the art and sound is just the same as the show, the story is pretty much non-existant other than the "stories" the mini-episodes have, and it's overall still fun to watch. It is just as ecchi, if not even more than the actual anime, so be careful when watching in a public area.
PS: "Borderline hentai" anime takes it far further than this anime will ever do. Don't worry about it too much, just wear a decent pair of headphones and you're golden.
