As a big Assassins Creed fan i skipped out on Unity because of poor reviews, and issues with the game, but recently i decided to give it a shot on game-fly, and my overall opinion on it is that it is a good Assassins Creed, but nothing special.
Visuals/Presentation
First off the visuals, which right off the bat you can see the stunning detail that was put into crafting Paris during the French Revolution. The buildings are highly detailed, and pretty to look at, as well as nice lighting effects. The presentation is also good in Unity, as i experienced no frame rate drops or slow downs through out my playthrough.
Gameplay
The Gameplay in Unity is rock solid, as expected from a AC game with minor tweaks to the combat that were made and in my opinion, the best combat to date for the series. It is definitely a more intuitive system then in previous entries. The new "parkour down" system is a welcome addition to the game and adds a lot more maneuverability to the game, and although it has some rough edges to it, I feel it can only get better in future entries.
Story
As far as story goes, Unity provides a decent quest and a couple of likable characters but ultimately the whole love aspect between Arno and Elise as the main theme just didn't cut it for me. The story was good enough for me to stay interested, but i wish they did more with the Templar/assassin dynamic, as well as some more modern era involvement. The best parts of the game for me were the time lapse sequences in which you have to travel through different eras in order to reach the end, they were interesting, and above all unique.
In the end Unity is a solid entry to the franchise but nothing spectacular, some key things were improved upon such as combat and the new parkour system, but the lackluster story is a bit disappointing and it has me wondering, and wishing that Ubisoft would take a break before returning to Assassins Creed, so we can all breath a little