Is the bundle worth to buy it?

Most bundles are more than worth buying as they support the developers. And most are so low in price that why even ask if it is worth it? If you are asking as per bundles on actual Steam, most of them are also well worth their price. But it all depends what you have to spend.

Game collector. So I buy any bundle if it's less than like 75 cents per game I dont own.

Interest, price and (how many bundles I already bought/how much money I already spent on games) this month.
In that order of importance. I rarelly care about cards cause I don't have the patience to farm them.

My personal interest, Steam reviews and Steam cards (even if I wouldn't like the games, I still would get some money back to Steam wallet). That's what make me decide if the bundle is worth buying.

I use these rules:

1. Look at my money for games this month, check if I don't reach limit (I don't have a lot of money for games).
2. I must don't have more than half games in bundle.
3. Games price in refs must be bigger than bundle price in refs (I know, it is silly ;D)
   (about bundle price: 1€ = 7refs(according TF2 key price); about games price: with cards 2ref, without 1ref, DLC 0.5ref, desura 0.3ref )

And then I can buy it.

I look at the bundles game and see how many games i want or would like to try. If the price of the bundle is equal or less then 75% discount on the price of the game i buy it.

I buy a bundle if it has games I'm interested in playing (the number of games to trigger my interest depends on the bundle price, total number of games in the bundle, cards, money I spent recently, etc). Or sometimes I buy "blind" as a gamble.

cost divided by number of games and then compare it with their historic lowest... If they drop cards even better

I used to buy bundles, but nowadays I'm interested in just one or two games per bundle and I just try to get those at Steamtrades

For me it's about: Is there a game in there that I want. If there is, with the prices going (even bundlestar's 5€ ones) the bundles are worth it. Since you get extra games that might surprise you/interest you/... it can be well beyond worth the price you are paying.
There has to be at least one I want or at least one game I'm very interested in, that's the only rule in the end.

Unless it feels like I own every game in the bundle, just buy them all. I do value my time more than trying to save a few cents or bucks with further evaluation and comparison on already massively discounted bundle games. Just doesn't seem to be worth the hassle.

Trekking through all the bundle sites every week once or twice. Checking what's new and which bundles already been bought. Hopping through the logins 'n CAPTCHAS and doing the dance of Amazonian Paypal. Checking which games are already on your Steam account, feeding the codes to Steam and waiting an unnecessarily long moment for the client to wake up again to eat up another code. Keeping up a list of used keys for some sites. All that is already time enough wasted for the matter.

Actually ... now I'm wondering, if it's really worth all that hassle either, even if the bundles would be free. :D Oh my, the games just get more devaluated.