I bought the Direzza ZII and so far they have been great. They are quieter than the OEM Bridgestones and the sidewalls feel a little stiffer in stock sizes.
I've taken then to Texas World Speedway on a hot day and found they will get greasy if pushed hard, however the will come back if you take it easy for a few corners. They are very progressive towards the limit, so I never felt on the edge, which increased my confidence in them, and brought down my lap times.
I have them installed on an extra set of OEM wheels and will use them mostly for track and getting to and from the track.
Here are some of my laps and as you can hear they don't squeal a lot like the stock tires.