Melbow - I usually gradually increase distance in new trainers.
(Obviously, you can increase faster if you're getting mild/no blister probs.)
By increasing slowly you ensure that you only get "latent" blisters near the end of the run.
You then get a "double-whammy" effect of gradually wearing the shoes in AND wearing your feet in (i.e. hard pads of skin will form in all the right places, thus naturally protecting against blisters over slightly longer distances)
If you find that you can't increase distance much at all without the shoes ripping your feet to shreds, then, yes, get rid!
Hope this helps/makes sense...?