I have not used Charm, either, but AFAIK you get full experience for opponents that a charmed creature kills (check your combat log if you can not see the experience listings during combat). When that charmed creature is killed, the experience you get is determined by the amount of damage you, your creature or a summon did; if your opponents take out the charmed creature by themselves you get no experience, if they take its hit points down 30% before the spell wear off, you would get 70% of experience if you killed it.
There is an experience bonus when defeating opponents at a higher level than you (at least in D2:ED it was a 1/6 bonus for one level above you, 1/3 for 2 levels and 1/2 for 3 or more levels above you). If you use Charm in situations to help with difficult opponents, then the bonus experience should often more than make up for that lost to using Charm. If you use it in normal difficulty fights, then obviously lack of a little bit of experience isn't really hurting your character.
You really don't need to worry about maximizing experience, though. There are similar debates about the usefulness of Mindread and Wisdom, but people have gone either way and done fine with their characters.
