As with all video games, Minecraft should be played in moderation. As it stands, Minecraft is actually a pretty harmless form of entertainment when compared to other video games a child could potentially play. Of course a child could find other productive uses for their time, but in the grand scheme of things this game is not so bad.
No I do not think that it is particularly a good use of time for kids. Kids could be doing something way more important with their time such as studying their school work, or being active outdoors physically because that is a big problem with children today. They could also be doing something good for the environment or for somebody else.
Minecraft along with countless other online games is never a good use of time for kids. Kids spend far too much time on games and not enough on practical everyday things. Kudos to Microsoft for cashing in on the decline of this generation. It will certainly gain them a giant cash flow above what they paid for it.
While Minecraft has more educational value that most video games, I do not think that it is a good use of time for kids, because I do not see my children being that creative with it. They would do a lot better drawing a picture than they would putting simple designs into Minecraft.