Once you get it working, it's tolerable and there are glimmers of gold in the giant pile of shit that Ubisoft dropped, and if it had come out after AC2.75, it would have been a fantastic game with a lot of improvements in graphics and game play.
That said, if you are playing on PC, take backups of your save files often. One little error or one little blue screen while the game loads (and apparently saves/syncs as it loads), the save game is kablooey. And you can only have one save game per account. It took me an hour to recover what I was worried was a totally destroyed save file which involved playing through the intro so that it registers the 'finished intro' save file and overwrite the new save file with the last good save file (which I had by pure luck on a rare database backup).
It's buggy as hell and even with the new fixes and optimizations, my rig which is a i5-4690K overclocked to 4.6Ghz, a Nvidia GTX 760, and 16 GB of RAM can barely handle it on highish settings at 900p. I've invested enough time where it's starting to get enjoyable, but I spend way more time on AC4 blowing up ships and storming Man-o-Wars. It speaks volumes about what my subconscious thinks.
Stick to AC4, ACU sucks unless you are a die hard completionist.