I didn't use first aid, but I did use both board and wards and Step II secrets. I thought that the combo of the two were great, but if you only want to use one, I'm not sure B&W is very comprehensive. It has a good breadth, but little depth. You'd need a lot of Q-bank or NMS questions to make up for it.
But, if you're going to supplement to book with another study aid, it gives you a very good and quick review of a lot of topics.
Just my $0.02
Good luck