i'm not a vet, but practical experience of one of my cats losing his hearing (aged 8) taught me that it seriously affects their judgement, particularly when it comes to crossing roads (dont worry my cat didnt get run over, someone came to the house to tell me my cat had taken to sleeping on nice warm tarmac and he was causing traffic jams) and we were forever falling over him because he lost his ability to know when he should get out of the way. in many ways he was a happier cat, it was like everything he'd ever worried about was invisible all of a sudden, he lived to ripe old age, but then we didnt let him leave the house on his own because he would have got run over. so in answer to your question, it is quite common and although it's serious because you have to take account of his lack of awareness for him, he can still live a very happy life and will cope very well with all the eating and sleeping business that cats want to do. :-)