I'll point out that I've had most of my idlers since the christmas steam sale last year, when tf2 was on sale, and you got a key for the purchase. And I've been idling with them on-and-off since then. So, I've gotten months worth of idling out of them, and have gotten well more than 110+ hats lifetime and hundreds of refined metal too. You just aren't going to be able to do that with just a regular account, unless you do insane amounts of trading. So, yes, I feel it's been worth it for me, but, I've also had my idlers for a fairly long time now, and value accumulates over time.
That said, the next-new-game-I-want-to-play
could get announced or even released tomorrow, and all my idling would suddenly become much less meaningful to me. Idling is only worthwhile as long as I still keep playing the game, for as long as I have friends who play the game that I can interact with. If I'm not playing, there's no reason for me to idle (since, I'm not doing it for the money or anything, and in-game objects in a game I don't play have little value.)