About now lots of Assassin's Creed Unity reviews for console versions of the game will be appearing all over the internet. We only received PC code a few hours ago, so our review is ongoing until I've air-assassinated every Templar in Paris and tried the new co-op mode. We'll have a verdict for you as soon as possible.
Sometimes last-minute review code can be indicative of port/performance problems, so we're loading it onto a few different machines asap to see how Unity fares. It runs well on my (admittedly quite hefty) main rig. I'm averaging 55fps at 1920x1080 with all settings on ultra, Vsync on and 2xMSAA. That's with a GTX 970 (overclocked a little), an i5 4670k and 8GB RAM.
That's certainly not the case for everyone. The Steam forums are filling up with players reporting poor framerates on mid-range gear. The other consideration is the fact that Unity is bound to Ubisoft's shonky client, Uplay, which is still downloading the Unity's 40 or so gigabytes of data. If you're running an Nvidia card, be sure to grab the driver they've put out specifically for Unity. Ubi also mentioned there'd be a day one patch that might help with bugs and some performance issues.
I've only just started with the game (you can play while it's still downloading), and have encountered some pretty minor texture pop-in on characters in rooms full of hundreds of people, but haven't hit any egregious bugs yet. Nonetheless, I'd recommend holding off on a purchase, especially if you don't have a high-end PC, at least until the port's been tested on more rigs.
Meanwhile, here are a few shots from the start of the game. If you're playing, do what I did after taking these shots and select the trimmed-down UI option in the HUD section of the main menu. It gets rid of the stupidly large console interface and lets you see more of the city.