The main part of this review is in 2 parts; first is written specifically for the OVA and the second is a snippet from my review of the main series with 1/2 tweaks.
After that there's an explanation for the scores I went with for the OVA compared to the main series but you're probably only here for one of the main parts so feel free to skip that if you want.

It's a good OVA and it's existence itself is fan-service to fans of the anime with the usual "fan-service" added to it.
I'd honestly love to get more anime of this series but as that's very unlikely to happen at this point one last "hoorah" for these characters will have to suffice.
If you like to watch your favourite maidens pestered by the usual "fan-service" OVAs tend to offer then you're in luck as there's not just one gimmick but a whole bunch of them including milk, school swimsuits, melting clothes and more.
Again, I'd love for it to have been more actual plot rather than "plot" but beggars can't be choosers...
(I myself don't really want to see my favourite girls being pestered like so but this is what you have to deal with in the anime community...)

Oh, and just in case you're wondering you will need to watch the main series unless you're some kind of barbarian that's only in it for the ecchi scenes.

Now onto the tweaked snippet (if the above was enough for you then you don't need to read this):
This is your fairly standard fan-service OVA which was bundled with volume 7 of the light novel shortly after the anime aired.
It includes a surprising number of fan-service "gimmicks" including milk, clothes in the process of being removed, school swimsuits along with ones that are similar to school swimsuits, melting clothes and more.
It has it's own mini-story/competition independent of the main 10 episode anime but requires you to have watched them as certain abilities, characters and whatnot are present that come into play throughout so unless you just want fan-service and won't watch the anime or don't mind being spoilt you should watch it after finishing the main series.

As for the scoring (seen on MyAnimeList but not here on AniList);
Unlike the main series I've given everything EXCEPT for the story and enjoyment an 8 whereas everything in the main series for an 8.
Story because it's your usual "fan-service" OVA BUT it does have it's own story/plot even if it is just to allow for the "fan-service" scenes.
The enjoyment gets a 7 instead of an 8 because "fan-service" 'isn't really my tea of choice' as I'd much rather have more plot or developments.