When I was growing up my Uncle had the Yakuza series, so I ended up playing a lot of them (Currently have Y1-Y5 done, not six, and have played 0 and Kiwami 1, and haven't played any of the spin off games/K2).
I was completely blown away by Y0. Storywise I think it really brings me back to the original Y1/2/3 games - imho there was a bit of a drop in writing quality in Y4+5. I LOVED being able to play as both a young Kiryu and Majima, I feel like it was a great insight into their characters prior to the first game, and it's really interesting to see bits and pieces of who they currently are in their past selves, Majima especially (it's also really interesting seeing the character breakdown through their respective fighting styles + heatmoves!). Majima's part of the storyline really hit me, especially with the introduction of Makoto and her plot. I know that a few people disliked the game because she was put as an important plot piece (along with her romance-ish thing) but I honestly loved it. I kinda lowkey ship it.
The ending hurt me real bad in the good kind of way, and I wouldn't change it at all. I think that Y0 was an amazing prequel series, and really kept the spirit of the Yakuza series while still staying fresh and new. It also really opened up the series in the West, which is great! Just wondering, why didn't your friends like the series? Was it their first introduction to the game? I know that a few of my friends were a little confused by the mass amount of side quests/collecting stuff/mini games to do, and felt like it was a distraction from the game, but to me it wouldn't be a Yakuza game without all of that silliness!