I've only played a little but I can already tell that Assassin's Creed Syndicate feels like going back to the roots (it reminds me quite a bit of Assassin's Creed Brotherhood) with some pretty polished gameplay besides all the eye candy.
The game has the Assassin's Creed missions and activities but it also has a Grand Theft Horse Carriage London 1868 vibe that is very enjoyable (it is just incredibly fun). The stealth feels tighter this time (building upon the changes introduced with Unity) and combat is definitely more rewarding, brutal and cinematic.
All in all I can safely say that it's a welcome addition to the series.
As much as I enjoyed sailing in Black Flag (and Rogue) but there is something to be said about exploring a huge sprawling city.
The Victorian setting is a plus especially with all the literary and cultural references (Dracula and Frankenstein are hard to miss but there are also references to the Cthulhu Mythos and that's never a bad thing). I may be repeating myself but the city of London is just perfect for an Assassin's Creed game.
Syndicate also conjures up a Gangs of New York vibe which may appeal to some players.
The only drawback I can point out so far is that Syndicate feels a lot less ambitious than Unity. I didn't have too much trouble with bugs in Unity (apart some embarrassing incidents) and Unity is clearly one of the most beautiful games I've ever played but I can understand why all those problems (don't get me started on micro-transactions and phone apps though) could have alienated some players.
My first impression is that Syndicate seems to be playing it safe, going back to the old formula that Assassin's Creed Brotherhood nailed five years ago. It's not necessarily a bad thing though and there is something to be said about the possibility of playing a male or a female assassin for a change.
Last but not least the main characters have that roguish charm that made Ezio Auditore and Edward Kenway such lovable characters, the thing that Connor completely lacked and that Arno Dorian lost halfway through Unity.
"Growing old is inevitable but growing up is optional."