Do mercenaries take loot or experience?

Mercenaries do not take a share of the loot but an active mercenary does take a share of experience as if they were a player group member. So if you are soloing with your mercenary they will be taking half the experience.

In order to promote groups of players over mercenaries there are experience bonuses for each player. A full group of 6 players will get approximately 22% more experience per kill than a group of 3 players with 3 mercenaries [Oct. 12, 2010]. This should be in addition to the normal experience bonuses for group size.

If you are able to solo with your pet and don't need the mercenary normally you might want to keep the mercenary suspended while you are fighting and only unsuspend the mercenary to refresh buffs as needed or to assist in tough fights. This way you won't lose any experience to the mercenary.