Yes, I am aware that this was written by the mangaka himself and slated as a sequel, but that doesn't exempt it from being fan fiction, imo. It adds nothing to the main canon (and actually detracted from it quite a bit), overtly glorifies certain characters (viz. the Generation of Miracles), and overall left me with the feeling that I'd have been much happier if I'd never even heard of it.

Let me start off by saying that I loved the original series. It may have been unrealistic in the extreme, but every single match was exciting and the protagonists' team, Seirin, is hands-down my favourite team in a sports manga. The anime adaptation was excellent as well. However, Kuroko no Basuke has always had a serious case of "misaimed fandom" : while written like a shounen sports story, featuring plain-looking, somewhat-talented-but-mostly-just-hardworking protagonists going up against the naturally gifted, egocentric and much better-looking antagonists, its fandom is predominantly female. Perhaps because of that, Seirin's one of the least popular teams, the entire fandom is built on the foundation of swooning over the GoM, and by the time the finals rolled around, half the fans were actually rooting for Seirin to lose. At some point, I noticed that the author himself had become biased in the GoM's favour, and while I can certainly understand why he'd want to cater to the majority, as a critical reader I can't condone it.

Extra Game is a prime example of Fujimaki's attitude. Despite having no relevance whatsoever to the plot, and seemingly for no other reason than to assure us that the GoM were still the gods of Japanese HS basketball, the very first chapter felt the need to point out that, following their legendary Winter Cup victory at the end of the main series, Seirin had lost the Inter-High for the third year in a row (thus again failing to make their goal, the Nationals). Some of the team bonds that had been built up throughout the course of the series proper were undermined in order to drive home the point, "in the end, the GoM is the best team," which actually contradicts what the original plot was implying: that they're better off as friendly rivals. Non-GoM were accorded diminished roles in the match while almost all the GoM guys were given yet more unrealistic level-ups. Add to all this the weird change in art style, the extremely one-dimensional and horrible "villains," and the fact that the movie adaptation, Last Game, went for an anime-original ending that ruined the second lead, Kagami's, in-series character development, and I can safely say that KnB has one of the most disappointing finales out of all the manga/anime I've read/watched.

**Tl; dr - ** Are you one of the tiny minority of KnB fans who liked the protagonists better than the antagonists? Do you feel that 30 volumes/3 seasons of glorifying the GoM's god-given basketball skills was quite enough? Then skip Extra/Last Game and take the conclusion of the main series as the true ending - I can assure you you aren't losing out. Or are you in the majority of the fandom and think the GoM is the best thing since sliced bread? Then you'll love Extra Game and its movie, so make sure not to miss them.