My son (9) has been really, really, into the Pokemon TCG for about a year now. It has waned once, but it is his main interest. He usually only has one, and he will live and breathe it for however long it holds his interest. We have been taking him to a league so he can play. My 6 yo is also interested, but mostly because he wants to do things with his brother and that is pretty much all he does. There are good things about it, I know there is a lot of learning going on, and the people at league seem to be really good people. There are adults there who organize and play it and we play at home as a family, too. So I like the community aspect of it.
There are some things I am conflicted about; I've had to really figure out what my approach to buying new cards and spending money on this "hobby" is; sometimes it's hard for me to say NO to them but eventually I am encouraging them to play with what they have and not think about constantly getting new cards. We also don't (and won't) do video games and we talk to the kids about why and they seem to understand our reasoning. The only other thing that bothers me is the immense amount of time and concentration put into what is a make -believe world, rather than our natural world (I have read a lot of Montessori so that is what colors my thinking on this. She says that our natural world is beautiful and amazing enough to not need to bother with fairy tales and make believe, something along those lines.)
Last night, we were out in the backyard late at night, and I gave the boys camping lanterns and they went up in the tree house and I could hear them reading a Pokemon handbook, comparing the sizes of different Pokemon. Before Pokemon, they would have been reading about dinosaurs, or sea animals, or insects, and right now they are not reading any non-fiction at all. I'm a little sad about that.
As a side note, I do think there are a lot of good things about this hobby, and earlier this month we attended the Pokemon National Championships (we live in Indy) and it was pretty fun (they did not play competitively this year but they would like to next year.) I just wish it wasn't the only, all-encompassing interest that it is right now.