After 4 months and 5 patches, let's see where Assassin's Creed: Unity is in terms of performance on a computer that doesn't meet the "bare minimum" specs (read: GeForce GTX 680). Hopefully this should satisfy your curiosity and help you make an educated purchasing decision.
Specs if you're curious:
PC: Lenovo Ideapad Y580
CPU: Intel Core i7-3630QM 2.4GHz (closest desktop match is an Intel Core i5-4690)
Memory: 16 GB
Graphics: NVidia GeForce GTX 660M 2 GB (roughly equivalent to a GeForce GT 640)
OS: Windows 7 Service Pack 1 64-bit
Game itself already has the latest 1.5.0 patch installed and all drivers have been updated. Graphics settings and screen resolution are at the lowest possible settings (see video for details). To get a slight increase in FPS, I have followed a performance fix guide below:
Please note however, that I have only followed the guide's points 1 and 2. I didn't turn on forced FXAA and hardware-assisted VSync; instead I opted to use Aero's native one (via the game's Borderless Window mode). Through several experiments I didn't see any noticeable improvement with forced FXAA or hardware-assisted VSync while in Fullscreen mode; so I thought it's best to switch both options off to try and keep the game running as smooth as possible.
For resolution, I chose 720p over 1024x768 stretched to a 16:9 aspect ratio. Sure, the game runs faster on the latter resolution, but graphics quality suffers terribly, while 720p is a good middle ground between graphics quality and performance.
The game runs at an average of 25-35 FPS per FRAPS. FPS will drop to 25 when around a significant amount of flora (e.g. trees, gardens, bushes) though hovers at close to 30 when in dense crowds. Cutscenes run at 30 FPS as well. The occasional stuttering and glitching you see in this video is not as frequent when playing the game without FRAPS.
I have since finished the main story missions and a few side missions as well, though I have yet to play Dead Kings. From what I've seen so far, IMHO it's not a great Assassin's Creed game. In fact, I think it's one of the worst entries in the series, performance notwithstanding. However I wouldn't count this out as it does bring some nice improvements to the table despite the drawbacks.
I recommend purchasing this game if:
1. Your computer meets the very high "minimum" requirements (for laptop users a GeForce GTX 980M is recommended)
2. It's on sale
3. You're a fan of the series itself or its historical tourism and abhor the modern day framing plot for whatever reason (prepare to be severely disappointed if this is what you're actually after)
Otherwise, the less popular Assassin's Creed: Rogue is a far superior product and is strongly recommended.