Two things to watch out for: they are prone to back problems (though not as much so as, say, dachshunds), and some bloodlines do have a "snappish" temperament and will bite.
The one that I have seen that was snappy was actually having a frustrated herding instinct, not remembering the difference between driving their handler and 'heeling'.
I saw a little girl with one in Obedience who nipped her ankles during heeling, so she wore thick socks till she got it stopped ;~D
My friend's dog was not anything like that though.
Do you know if there's a great difference in personality between the Pembroke and the Cardigan?