I recently started using it... and I like it!
After months of using G3 in default and classic modes--and on occasion defaulting to Cinnamon--I still find it hard to buy into default G3 methods. Classic G and Cinnamon at the end of the day, are about getting into a G2 way of working albeit with G3. I can buy into those alternatives... but they're not stable enough and dare I add, not user friendly in the customizing department.
However... recently I had the opportunity to play with both Ubuntu and Xubuntu 11.10. Being a ex-XFCE user, it was easy to be impressed with Xubuntu but I digress... back to Unity. After playing on the live CD, I made a partition and for now I dual boot with Ubuntu. I don't yet get what the fuss is about. Ok. So their 'docky' is a 'dash' and instead of sitting horizontally it sits at the left vertical. It doesn't look cool... but it works. And their dash button--imho--works better than the one in G3. What "sluggish!?" It has more features than G3 dash like for instance, it remembers what you used... kinda like a recent shortcut history.
And yes, it is customizable with ubuntu-tweak. Its auto hide-able... change sizes and opacity, etc. While ubuntu-tweak doesn't have an option yet to change the icon, I have seen solutions for that as well or even placing it at the bottom rather than on the left. All in all I find Unity to be full of interesting surprises... that just work for me. And... you don't have to use the dash--but as I said, it has got some features that G3 lacks--try classicmenu-indicator instead... nice!
I guess at the end of the day... while Cinnamon is the way to go and hopefully it'll be a--default--session in Mint 13... for now its early days. Unity was out there before G3. It seems to me that Mint devs should've been making extensions in this space rather than fuss with G3 and reinvent the wheel. After all the rest of the Ubuntu repositories still work, apps work and for me, after some system and Nautilus tweaks, this has that familiar look-n-feel of pre-Mint 12 days.