i have one salt lamp given to me as a gift.

it gives a nice glow to my room. ^^

Its good because it acts as a natural air ionizer. It release negative ions just like electronic ionizer just that the amount is lesser.

Salt lamp need not be on (with the ligh bulb on 24 hours) to be effective. It will be effective with or without the light. The main purpose of the light is the heat released to dry up the moisture or to prevent moisture from reacting with the salt. Due to the humid conditions in Malaysia, salt (sodium cloride) absorbs lots of water just like the the "thirsty hippo" or dehumidifier (calsium cloride). Therefore if there is no heat present the surrounding areas will be full of salt water. I forgot to on my salt lamp and went off a few days and my living room is fool of salt water. It will cause electrocution too.
To reduce the amount of salt water, the room must be well ventilated and on the lamp at intervals. Putting salt lamp in air-cond rooms will also reduce the water but make sure the air-cond is on for a long hours.

Therefore it is always wise to wrap up the salt lamp with plastic bags if you do not intend to on the lamp for a long time.

One more piece of info:
the edible salt we take is natrium cloride
natural salt rock consist mostly of sodium cloride. The higher the purity the better and the more expensive.