They'll make OK cows, but as others have indicated, depending upon your management scheme and level of nutrition, some may be 'hard keepers'; may milk themselves down to nothing and be hard to get re-bred. A big calf is one thing, but if you only get one every two years, you're losing money feeding 'em.
I started my herd with a bunch of Holstein heifers, but nothing still here has more than 1/8 Holstein in their background, and not more than a couple of them - the others have 'culled themselves'.
I'd have some concern about the possibility of introducing Johne's Disease(see recent thread in the Health and Nutrition area), as it's pretty common in dairy herds, and even though these heifers may look good now - or even at 2, 4, 6 years of age, if they're infected, they could serve as a source of infection for their own calves, as well as other animals in the herd.