The effect on experience gain is 10% per affinity, stacking up to 5 times. It would be reasonable to assume that the maximum possible skillgain accrued from affinities (at the maximum stack) would be in the range of 50%.
An affinity in main [[Fightingfighting]] skill does not seem to have any effect, except when skilling up to 20 with a practice doll.
If you lose your affinity, you will get a new one in a random skill in around a weeks time. (This only occurs if you have a single affinity which has been lost).