The addiction to computer games can be almost as difficult to break, especially for young people, as cocaine. Numerous studies have come to this conclusion. Also, your mind idles only slightly more on computer games than T.V., so it is not the place to find intellectual stimulation. It can lead to social isolation, and as for being fat, I can eat two chickens a day, and if I play computer instead of working out, it brings on fat fast. I wasted a lot of my early life on computer games, due to my unique problems in my environment, that made me start on this to escape from the world, which later made the decision to take other escapes more normal. It eats up your self-determination, because you don't need to chase actual enjoyment in life when you can use computer games as a poor temporary fix. It is a waste of time, and there have been countless lives spent on trying to beat the same exact games, to no benefit whatsoever to anyone, though many work at games like they are a job.
That being said, they're only marginally more addictive than movies, and if you live in a normal social situation, it is less likely that you become addicted. I have known someone who was abandoned by his parents, and lives his whole life on final fantasy and the welfare line, and the occasional medical testing. It is amusing to play games in moderation, which we all need, but the games will make it so players are almost always encouraged to play more or play the game again and again. Games do little to stimulate imagination, unless you are the designer, but can be an excellent tool to introduce new ideas and technologies.
In sum, computer games can destroy your life, and an addiction to them is extremely dangerous, as it will take up your life, and either you will enjoy it, or you will get to the point where you couldn't possibly imagine anything else to do. Think of an hour on a computer game as a glass of liquor, and don't abuse it.