Like Rai said it is teh best source of tanning as it protects your skin and you don't get damaged from the high amount of UV exposure but it isn't good either. For one it looks horrible and nasty for two sometimes they contain vicious chemicals in and weird liquids which can make your skin go inflamed or sore and you could get a rash and such.
They're the best kind of tan you can get. Laying in the sun will give you sunburn/skin cancer, same thing with with tanning beds. All that UV lighting is bad for your skin.