It's performing about the same speed - some programs open a bit faster, and it takes about half as much time to boot up as Vista did.
The new interface - wow. Basically, the Quick Launch and Taskbar ideas have been fused together. Now you have one little square-icon thing for each program open - clicking that icon opens all windows associated with that program (all chat windows, Steam windows, etc). You can also right click and select the option of permanently pinning the program there, making it a Quick Launch program.
Other than that little bit of awesome, the interface overall looks cleaner, is easier to navigate, and (from what I've heard) actually takes a bit less memory to look good than Vista's interface does.