Is Minecraft any good? My son, aged 6, wants it for Christmas

Depends on what you consider good, actually I world consider it quite advanced for a 6 year old to get anywhere with, he could get bored within half an hour or so.

Minecraft is a creative sandbox game. In it you have a very large world which you can deconstruct to get parts for making your own things, like buildings.

There's no story or plot, it's basically "Explore, Gather, Craft, Build." A character has infinite lives, but drops everything they were carrying when they die (and if not collect soon enough, those items disappear.)

There's no blood, or human enemies, but there are lots of zombies, giant spiders, and other sometimes spooky monsters.

There are additional game modes that remove the monsters, or allow unlimited cost free block placement letting you build anything you like.

There are many, many, Youtube videos available for the game if you want to see some more of what it is about.

I agree with Thomas, it's alright, it's a creative game, you can build, construct, destroy, reconstruct --- It's, as far as I've seen, "virtual Lego".

Although I grew up with the physical Lego, I can't say anything against Minecraft, but make sure you block the in-program buying options. --- Although I loved the logical switch parts in Minecraft, which are leading young people to electronics. I spent a whole night playing with them.

Yeah it's good, easy to pick up for kids and fun to play. It is literally on every system now

Yeah. It is kind of a do whatever you want game. He can build thing and try to survive the nature. Many just don't like it because the community around it is not the best, but it's a great time for kids.

Minecraft is a very creative game. However, it is also very addictive and I would therefore advise you to limit his screen time to prevent headaches (as I have experienced!)
You obtain resources and place them in order to create buildings and shelter to survive. You also need to manage your health levels to gain XP.
Be careful though, because he may play on servers where other users can message on servers so ensure he knows the dangers and precautions: never give out personal details… Blah blah blah. And whatever you do, don't go on his world, don't move a block, and don't carry a lava bucket around whatever you do. Hope this helps and prevents any future fires and deconstructionism. (RIP that hotel)