I've got one of the Hauppage variety - picture quality just as good as a TV - though I'm using a portable aerial so it can be temperamental (esp. Ch. Five).
I would say that if you go for it, make sure it supports Nicam stereo - mine's mono - it's ok but a little dull. Some also allow you to see Telexext and capture the data thereof - can be quite handy sometimes.
A final note is that if you are connecting it to an outside aerial I would hook it up to to a surge protector...just for insurance.
Cheers
amofw