Unpopular opinion: It's really not as bad as people often claim it to be.
At first, I thought it was some bias I held just because it was the first game of the series I've ever played when it was new, but now that I've recently played it with a friend again, I just have to say it wasn't all that bad. That is not to say that it was perfect or anything, and it nothing compared to entries like Resident Evil 4, but it's not too bad. Way more fun than Resident Evil 5 in my opinion. Though I do understand that those looking for the classic Resident Evil survival horror experience won't be happy with it, as the game is way more on the action side, compared to previous titles.

What I like are the intertwined stories of each campaign. These consists of Leon's & Helena's, Chris' & Piers', Jake's & Sherry's and Ada's stories from 2012 to 2013 and take place in Tall Oaks, USA; Edonia, East Europe and Lanshiang, China.
Chris campaign probably was my least favorite as it was mostly all action and that it probably felt most like Resident Evil 5, which is my least favorite game in the series. His story starts in Eastern Europe after his troop gets killed due to a lady that appears to be Ada Wong. Working for the BSAA, he and his buddy Piers are trying to stop BOWs and prevent some acts of bioterrorism.
Leon's campaign, while still being filled with action, has also some quieter moments and is probably a bit more aligned with the classic Resident Evil feel. It starts with the president dying due to an act of bioterrorism, which he is later accused of. It's all about finding the real perpetrator. And in true Resident Evil fashion, it even ends with a rocket launcher!
Jake's story is all about Wesker's son, who has natural antibodies to the C-Virus, which makes him valuable in creating a vaccine. Jake and Sherry have their own little Nemesis alike that follows them through their campaign.
And lastly there is Ada Wong's campaign which ties all campaigns together and answers various questions. This is probably most true to the Resident Evil formula, not being as action filled as the other and more puzzle focused.

Gameplay wise I think the combat is quite good and Co-op (which I only played recently) is also fun. Now, there isn't really any inventory management unlike the previous titles, but it suits the more action themed entry a bit more.

Now, as already said, it's far from perfect. The levels are really linear and there are no documents to be found for more lore. Instead, you have to destroy blue medals all across the levels, some exclusive to some characters, to unlock extra lore in the main menu. And while I liked Co-Op, it was cheaply integrated into Ada's story, which didn't have Co-op at release. The second player is some random soldier, with no name or backstory. He does not appear in any cutscene and worst of all, he can't really interact with anything. You can't interact in puzzles, you can't do a lot of the quick time events to progress, you can't even open doors(!!!). The real player 2 treatment.
Now comparing this with other entries of the series, it's one of the weaker ones or mid at most, but looking at it as just a game, it's quite some fun. And better than 5!