Obviously they are a fairly quick car in comparason to an everyday family car, but they are certainly not superquick.
My experience relates only to an old 2.7 version however, which I once had a coutesy car. The new bigger engines pump out a bit more zip.
I would seriously opt for a Honda S2000 however. Yes they don't have the Porsche badge, but I would argue (along with Mr. Clarkson!!!) they look better, will be more reliable, be cheaper on petrol, has a quicker roof and revs to 9000rpm.
And to further quote him, which is so true, "Boxsters are merely for people who can not afford a 911, but want one so badly"