I've been using it since the first Beta & have had no issues worth mentioning so far.
You couldn't compare it to Vista in so far as Vista was like trying to drive double Decker bus with a one litre engine, whereas, Windows 7 is like driving a one litre car with a 4.8L BMW engine.
It's very hard to put into words, you just know when you're using it, that it's smooth and responsive as opposed to jittery & sluggish.
Remember, Win7 has a footprint not much bigger than that of XP, it's nowhere near the resource hogger Vista is/was. Because of that, it will run happily on much lower Speced machines too.
The bottom line is that it will be a huge success compared to Vista.
Remember too, all new OS's take time for the user to get comfortable with them.