I have found it to be a great investment. I own the cards, and people can pay me to play with them eventually. But I normally combine them to increase the levels of them so that they have special abilities before I sell them. I was lucky to get most of the alpha cards before they ran out.

I know people who invested $4000 and their cards are now worth $8000 because they are high ranking within the game.

The game itself is simple enough (and safe) for kids to play, which was the point of it.
You don't need complicated rules, you just have to understand what trample, flying, poison, etc. do and what the individual card stats say.