This is pretty much the ur-example of its type, which most other porn games of this variety aspire to.
Summertime Saga, on its own merits as an example of its subgenre, is a 5 star game. But, in the greater context of its release schedule is probably a 3.5 or 4.
So, as for the game itself? It's what I call the "Suburban Harem" subgenre. The protagonist is always an unassuming everyman. Physically lean, unoffensively attractive in a boy-next-door way, with a boring haircut and boring outfit. His personality is generally kind and thoughtful, of average ability and intelligence. His horniness conveniently lands him into all sorts of sexy hijinks. So, par for the course, he's unexceptional in every way except for the size of his dick, which is an absolute unit. He lives with the classic Doting Milf archetype, and the bitchy Hot Sister archetype. There's no father figure around, and as the protagonist steps up his presence as Man of the House, the sexual tension that's already at a 99 out of 100 boils over into sexy schenanigans.
What sets this game apart is the cohesive, gorgeous art style, humorous and endearing writing, memorable characters, and plotlines that actually engage beyond contrivances for sex. It also has a strong through-plot about your late father being bumped off by the mob; this framework narrative is something many of its contemporaries and rivals lack.
The adult content is absolutely stellar, being some of the most consistent in the entire industry of porn games. There's a plentiful variety of girls for the MC to dick down, from the two aforementioned (totally not) mom n sis, to the artsy scene chick (along with her artsy tattoo artist sister and their busty goth friend), to the Mean Girl bimbo crew, to your friend's hot milf mom next door, to the religious girl next door, to stand ins for Melania and Ivanka Trump, to actual monster girls, to gmilf inspired by Pixar side characters of all things. Most of the scenarios are in the vein of "Teen T&A" movie, except taken to an X-Rated conclusion. If you're into being a big-dick everyman that literally every woman wants a piece of, you can't do much better. There's even a trans gf option, and there's gay content in the works, if that's up your alley.
The game's overall structure is a point and click sandbox. I'd describe the content as "clusters" that occasionally bounce off each other, with recurring characters moving and appearing in various viginettes, which helps make the world seem alive. Various groupings of women are gathered around a plot thread, such as the artsy girl, her sister and their goth friend being confined largely to their own path, but mingling with a couple other characters briefly. Most games of this type feel very shallow, but here the world feels vibrant in a way I haven't seen done in any other example of the Suburban Harem subgenre.
The actual gameplay itself is simple point and click adventure stuff, with occasional simple quicktime events and minigames. There's a typical time-state mechanic that's pretty common to these kind of games. All in all, it provides a skeleton to see the sexy art assets and enjoy the romantic/erotic stories, which is why everybody shows up.
HOWEVER
If you're unfamiliar with this game, be forewarned: The release schedule has become a problem. As of this writing, it's been over a year since the last major content update, with an interminable drip feed of tiny, minor updates months and months apart, and no hard date set on the next one, which is merely a technical update, not a content update. Now, for most smut indie games it'd be distasteful and rude to comment on release schedules, but Summertime Saga pulls in a bigger cash budget (via crowdfunding) than most indie devs producing more intricate and ambitious games could ever dream of, it's not wrong to expect more from this developer. They have the resources, they should be making better progress by a long shot.
Given the basic gameplay of this project, it's not what I would call a good look to take this long to release content. I'm not sure what is actually holding back this project from being finished, and simply crowdfunding a remaster or simply starting a new project on a stronger foundation.
If this was a minor thing, I'd give STS a 5/5. But it's really not prudent to ignore it. Play it, but don't pay for it.