In large doses it causes a very deep sleep, from which it is hard to arose the sleeper. This is not a coma. In fact the GHB user is likely to waken feeling refreshed and alert. However, for moderate use, GHB will probably not be habit forming. One case involved a man who took about 20 to 30 times the normal maximum dose.
High doses cause vomiting, which tends to limit the total amount that can be ingested.
It is a naturally occurring nutrient. The stuff on the black market may contain harmful impurities. A large dose can put you into a deep sleep fast . The Cognitive Enhancement Research Institute's GHB page. A good list of articles and links.