Disclosure: this review is based on the perspective of a single-player gamer, and of someone who is a fan of the anime series the game is based on. I have only seen the anime and have not read the manga.
In case you missed it from the trailers, Seven Deadly Sins is a fighting game based on an anime and manga series with the same title. The game is made for people who enjoyed the original content and thought: 'hm, I wish I could play as these crazy characters and do these epic fights' and/or 'I wish I could somehow experience this fictional world that I like so much'. As such I see this game as a companion or complement to the franchise, rather than as a stand-alone thing. I think that's the best approach to this game.
The Adventure mode is a chronological re-telling of key moments from the series, and unless you are familiar with it you will be lost and confused because the narration is very patchy. The developers fully assume you have seen or read the anime/manga. People new to the franchise would have benefitted from a significantly larger set of cut-scenes and a more explicit narrative. This was not an issue for me since I already knew the storyline. Being familiar with the series I also had an existing liking for the characters, and they are represented really well in the game, with lots of voice-acting and appearances.
Worth pointing out is that the game's narration is not a 100% match with the anime, they've changed some of the settings and wordings to suit the game. They have also avoided all the gory scenes and certain "sexy" ones. It's still an adult game however, as there's enough fan-service to ensure that (in tune with the anime itself). As expected the game does not include every scene and event from the anime, just those that are needed to drive the main storyline. I would estimate that the game covers about 30-40% of the anime's storyline content.
The bottomline is: if you haven't seen anything of the franchise, you will definitely be lost to what this game tries to create. Not that everyone who liked the anime or manga will like this game of-course, but it is a prerequisite.
Being a fighting game, it should not be surprising that the single-player (Adventure) mode revolves around combat; you go from one combat scenario to another. That's what 90% of the game is, and that's what 90% of all combat games are. The remaining 10% are dedicated to storyline content. The game is semi open-world, as you can travel freely in each unlocked area, doing various combat quests. Most of the quests feature the two main protagonists, but you have plenty of opportunities to play the other characters in the ensemble, as you build your squad. You can make magical "mods" that can be applied in different ways for each character (health, damage etc.), creating different "builds".
The complexity of the combat was on an ideal level for me. I don't play that many fighting games, mainly the One Piece series, and Seven Deadly Sins is on the same level of complexity as those games. In other words, rather forgiving for semi button-mashers like myself. You have three regular attacks (weak, strong, ranged), three magical attacks or boosts, a super-special attack, and jump/dodge moves. It was complex enough to be challenging and enjoyable, at least for someone on my level. As you collect more characters to play, you end up with a highly diverse range of combat styles. I had fun returning to older missions in order to get a higher score. Those who play a large number of fighting games will probably find the game too easy, especially if you play more complex and technical ones like Guilty Gear. However, I don't think the game is aimed at these types of gamers.
You will have to repeat some of the combat missions in order to get a higher score, since this will unlock more quests, needed for progression. However the game did not feel repetitive or boring to me, since I really enjoy this fictional world and the combat. You can also choose which missions you want to re-do, and they re-spawn on the map rather quickly. You can easily return to them once you have improved a character, by boosting their attacks for example.
The fun of the combat was enhanced by the destructible environments; rocks, houses, trees, and so on. The combat becomes epic and very cool as you learn to master the magic- and special attacks. However I do wish the combat arenas were bigger, as they felt somewhat small compared to the anime.
Visually the game looks great, it's colouful and lively. There's lots of detail. The music too, is excellent; it's upbeat and energetic. These help to create a positive, inviting and cheerful atmosphere everytime I visit the game. Combined with enjoyable combat, a fun gameworld, it makes for a game that's hard for me to put down.
To conclude: this game is very much designed as a complement to the anime and manga series, as a new way to experience the tale. As such it has a rather narrow consumer target. However I do think games should be allowed to target a specific group of consumers without being punished for it. This is a niche game, as it will mainly find a home with people who've enjoyed the franchise and who enjoy a more casual fighting game. Having a tighter and more detailed narrative would have greatly benefitted the game, even from the perspective of someone who has already seen the anime. What you get is a game that feels solid at the core, in the gameplay design, but where the narrative feels too fragmented and random. Overall, I got what I expected and wanted from the game, I'm 80% happy with it.