For our non UK members here are some thoughts from Autocar and EVO which are IMO the two best car mags available out there (excl. Auto motor und sport but I don't read german).

Autocar:
Says it's a compromise with especially a noisy roof.
Not as good as the coupe but compared to the competition (Jag, SL, etc.) the best car here.
In short: "For drivers, this is the best conv. less than Pounds75,000 will buy, but the coupe is both better to drive and better value."

EVO has much more praise:
"Despite the apparent conflict with the Boxster, it is a different, more vivid experience. And while the core appeal of an open top 911 might be lost on some of us, if you're a fan of open top perf. cars, Porsche again defines the breef."
So basically, almost as good as the coupe with the bad point here being the questionnable image of a open top 911, in other words, nothing.