Rudolph wrote:stay away from the cheap ones.
I've had two wich reversed polarity on me which is disconcerting. never had that with a silva. I've no experience of karrimor.
I once had a Silva reverse polarity on me. We took a bearing on the top of a corniced Beinn a'Chaorainn in a whiteout, and my compass pointed in exactly the opposite direction to everyone else's. Fortunately there were at least two other people in the party with compasses with the correct polarity, so we followed the bearing given by the majority.
But don't let me put you off Silvas. They're good compasses. I think I just put it too close to the unshielded speakers in my old car.