he could be a good actor when he could think of someone to imitate - for Being There, it was Stan Laurel. But he got badly up himself. (There's a film about him starring Geoffrey Rush which seems pretty accurate: announcing to his shocked family that he was off to marry Sophia Loren, for instance.) When he was making Casino Royale, he insisted on a huge paypacket and a Rolls Royce, insisted on hiring Orson Welles - and then fell out with him because Princess Margaret turned up on set and spoke to Welles more than Sellers. He refused to be filmed at the same time, so their casino scene had to be shot twice, once with each actor, Then he vanished and they had to patch his performance together from offcuts. He didn't have many friends left in the industry when he died.
Early on, though, when he was playing three roles in Doctor Strangelove and The Mouse that Roared, he was very good.