Well yes, it is free monies, I have 5 cents. But it is steam wallet 5 cents, so its not exactly that great.
I've made about 20$ on steam cards. All you have to do is leave eligible games on and let the free monies roll in.
Yup. Cards are awarded based on time spent playing, because any other stat could be manipulated and might encourage awkward playstyles. The cards themselves sell pretty easily on the Steam marketplace - you can only earn half the cards needed for a single badge by playing the game, so if you want a complete collection (or want more than one level), you pretty much have to buy them.
Honestly, I'd prefer a different implementation. I think it'd be more interesting if people could earn an unlimited (or at least large) number of cards, but only specific
types - instead of being limited to 4 TF2 cards, I could earn unlimited copies of 4
types of TF2 cards. So I might have lots of Heavy, Medic, Scout, and Pyro cards, but I'd have to find someone to trade those with for the rest of the set (and I'd have lots of duplicates available to trade). As it stands, the only way to get a complete collection is to buy cards, trade non-card items for cards (same as buying them, really), or pray for multiple booster packs (and I haven't seen one yet). But sadly, I don't work for Valve.