I've got a CP 21mm, same difference really. I've noticed a few differences than the PHBG (which I swear by):
1) Turning out the fuel mix screw makes it leaner (not richer like the PHBG)
2) Where a 95ish jet is for a PHBG, it is a 83ish on the CP (for a 50cc 10k+ rpm setup)
3) Dunno wtf the black retainer piece is for inside of the carb (I left it on)
4) All aluminium body, lots of o-rings instead of gaskets. Lighter and better.
5) Very, very nice pull choke. Way better than the PHBG in my opinion.
You can always look at Polini's own document on it http://www.polini.com/en/page_696.html
My personal opinion is only left to the few days I've ridden with it installed. First impressions are they have a lot of technology for something so simple. That isn't to say it is a bad carb, it's to say, there are a lot of things that could make tuning go wrong: lots of jets inside the carb and on the venturi, weird overflow valves, etc. For racing, sure, you might need that, but not really on a daily.
Downsides to the carb are that it's not a flat slide, bit more difficult to find jets/slides/parts, screws strip easily and require loctite for the more vibrating engines. You'll also want to polish the venturi to a mirror shine since it isn't smooth out of the box (a good practice to do to any carb in any case).
I won't tell you to stay away from the carb, but I will tell you the PHBG is much more well documented and predictable in terms of what it will do if you change something.