If you're already familiar with the main story, then suffice to say you're already expecting what the basic story entails. So if you can't stomach that sort of stuff then this isn't for you. With that out of the way let's see how this spin-off does. Note that this review is based on the first episode.
In general from what I've seen I could say that there are some areas in which I felt improvements have been made over the original series, and other areas where it does fall short off.
One thing that disappointed me was the sex scenes. From what I gathered, out of
a 16 minute run time only roughly six minutes were spent on the hentai aspect of a hentai show. This time was spent on five different scenes featuring the main female character Reina of this episode, so you can already tell that each scene had barely enough time to get focus.
That said, there was some interesting variety in them, from the situation to what Reina wore, aside from two scenes where she wore her work clothes. Casual, work, pajamas and none at all. Plus the variety in the sex does help, especially when there's not a lot of time on each scene.
The rest of the time was spent building up the plot. This is where I believe they did improve over the original series. The plot here is more in depth compared to the first series, and the male character even references to the male character from the original work, suggesting that they are in the same timeline.
Animation wise it's what one would expect from Mirano, it's not perfect however the movements are well done and none of the sex scenes felt stiff any way.
For characters, the male character wears a mask on throughout the entire episode. It's never explained but maybe it'll be brought up in a later episode.
From what we've seen of her, Reina's mom Shiho doesn't look as much of a MILF compared to the mom in the original series, if anything she looked more like an older sister. We also get another female character, the maid Sheiri, though not a lot to go on about her other than the size of her breasts.
Overall I'd say it's not a bad beginning to a spinoff, however I do hope that future episodes give more hentai scenes.