I think it depends on the child.
My two girls play Minecraft and have been quite obsessed by it over the past two years (so aged 8 and 10 now). At times this has been too much- we have had to set up boundaries around use and even then my oldest gets extremely into any game she's playing (she is now allowed to play on the servers, my 8 year old isn't) and can be quite verbally aggressive when asked to get off- this then results in a threat/actual deprivation of minecraft the next day. Only being asked to get off a computer game would get this response though- which is why I don't like them very much. They also eat their food quickly so they can get back on the game- even if I set a curfew, because that means there's less time for Minecraft.
Yes, they do have friends, go to school, watch TV and read books, but there's no doubt that the Minecraft obsession is a bit much at times, as even if they are not on it (due to my rules) they are thinking about it!
It's more one of mine than the other, one plays in creative and builds nice farms with ponies on them, the other plays warfare type games on the servers- so everyone saying it's creative is only right up to a point, you can choose how you play and not everyone wants to build nice little buildings- as I say, one of mine plays in the other mode or on survival games- there's no blood but they are still basically about who dies last!
I wouldn't say don't use it, but I think some of the replies might relate to younger children, your child is only 6 and I'm sure it would be fine- once they start getting towards adolescence and issues of socialisation/staying in the bedroom occur, I think having too much time on games can be a negative. It is difficult though- how much control should you have over a 10/11 year old's down time? I do set limits on Minecraft otherwise I really think it would take over our lives!