Haha, McCartney is a god for his bassery alone. It's often not until you play bass and learn Beatles lines, that you realise how good is he is. His style is slightly odd in that he doesn't seem to use a lot of slides/hammer-ons etc... he has an odd shuffle and a nice subtle groove to his playing. Like others have said, his basslines are perfect. Change one note and the whole song is ruined. McCartney's lines can be fairly complex too.
And yes, I suppose the fact that he sings at the same time makes it even more impressive.
And yes, I suppose the fact that he sings at the same time makes it even more impressive.