I enjoyed the game a lot, especially for the twin-protagonists angle, which I often wish there was more of in RPGs, and the high-quality Victorian London setting was fun to see, especially as a Londoner (even if the accents were a little cringe at times). I also enjoyed the story a lot and thought the characters were strong, especially Jacob and his fruity little narrative with one of the villains (which I desperately, desperately wish had been explored more, it was a huge hook for me and a fascinating kind of formula-break when the protagonists started drifting apart.) The gameplay was fun and dynamic, traversal and stealth mechanics were fun, beating people to a pulp was fun, going ham on horse and carriage was fun. I found the world genuinely engaging and always got sad when I found one of the immersion-breaking sci-fi boundaries to the edge of the map.
(I could talk a little about my dislike of the weird time-travel future-past back and forth thing and how much I feel it jars, clashes, cheapens the narratives, breaks immersion and engagement with the game and its protagonists in their historical settings, etc, but that would be a bigger Assassin's Creed problem rather than just a Syndicate problem, and thankfully in Syndicate it was for the most part pretty easy to ignore.)
This was the only AC game I've played in it's entirety so I can't compare too much between it and other games, but I did feel that Syndicate was solid and fun to play, both in combat and exploration. Jacob's individual plotline and the dynamic between the twins was really strong and a powerful incentive to keep going and see where their story would end up. The graphics and especially cutscenes (character faces, etc) were really great and made it very enjoyable to spend time in the game, the world was very immersive and the characters felt real.
The Ripper DLC was enjoyable with an interesting premise and I found playing as the villain very unique and intriguing, though there was one massive criticism I had for it which was the cheapening of mental illness - namely, a sequence where you go through an asylum and have no option but to 'kill the crazies'. Given the overall left-leaning and progressive politics of the base game, this felt like a really cheap move that was really disappointing to play through and felt very at odds with the overall vibe of the game, like a pot shot for horror points. More something you'd expect in a cheap shootemup rather than an RPG that tried to be and for the most part was thoughtful in its writing. This combined with the insanity angle of the villain you spend some time playing as just felt a bit gross/exploitative and lowered my opinion of the writing overall, at least for that DLC.
Otherwise a good game, I do often wish there was more for these characters.