My F4 has it and I love it. It doesn't have the travel of a fork style. It is very adjustable. When setting my bike up on purchase my LBS called Cannondale and found they could really put the pressure in it to handle the heavy weight (I am 300 lbs). My experience with fork style shocks (Rock Shox) is limited to the test rides I took on other bikes while searching for the right one. The thing I like most is that the lockout knob is on the top of the shock up in the center of the handlebars as compared to down on the top of the fork. Much easier to reach and lock out.
If you are looking for a shock for a hardtail that gets some gravel roads and small roots and rocks then it is the way to go. For long drops and big hits the fork style with longer travel may be what you want.
Reading about the shock before purchase said the cons may be the bearings. Locking it out and smashing the bearings may cause premature wear. I have found that it's short travel causes me to lock it out less than I did on the fork style with longer travel. So it may not make much difference. But it is something to be aware of.
I am not a fan of the Head Shock system but to each his own. I like being a little more universal as far as being able to switch out my fork for another one if i am in a pinch or run across a great deal. As far as performance goes lots of guys i know or have ridden with like em. Any specifics you really want to know?
Tajcrews brings up a good point about the lack versatility of it. If you have a rock shox or standard suspension fork and break it you can swap any other type of fork in depending what may be available. If you are set up for a headshock you either get a new headshock or you have to get an adapter to be able to use other types of forks. So you don't want to break without a Cannondale shop nearby or an LBS that has the adapter, or you will be waiting for parts to be ordered.