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The Humble Bundle website is VERY RESPECTABLE & LEGITIMATE as they always seems to pry money out of my wallet every week or two with their deals (that I actively partake in)... & I've been picking up bundles since "Humble Indie Bundle 2" & Bundle 11 is my 42nd purchase there (don't mean to brag, but just using it to show my experience & confidence with the site).
The Humble Bundle site is VERY FLEXIBLE with payments & pricing (except for their store... those are hard-set), as they offer 5 ways to pay!
* Direct w/ Credit Card
* Via PayPal -- Good for those w/o a credit card (may need to deposit funds into account first prior to purchase)
* Via Amazon Payments
* Via Google Wallet
* Via Bitcoin -- If you're into Crypto-currencies
If you're concerned about security, going through PayPal, Amazon or Google would be your best option as you're protected by those payment processor's policies. While it takes a little longer to process your purchase (~15-30 seconds, depending on your connection) with them, it's still completely safe.
As for being worth it? As long as you jump on the bundles, you're getting the games for practically a steal (up to 97% off... assuming you beat the average. Can go as high as 99% off, if you go with bare minimum of $1.00. The site notes how much the bundle retails, so you can do the math) for TWO COPIES of each game in most cases: DRM-free (which you can download for any OS at ANY TIME) AND Steam version, unless specifically noted (The site will clearly note any restrictions, limitations or exceptions in red BEFORE you set your own price). The site even notes the $1.00 requirement for the Steam Keys.
The Humble Bundle site has been running for a couple of years now & have been backed by big name developers... EA (with their Origin Bundle), Double Fine, Team 17 (makers of the Worms series) & Taletale Games (makers of The Walking Dead & other point-n-click adventure games) to name a few; therefore, it's safe to say that they won't scam you.
How their system is set up (with their deals & bundles), you may feel like you're actually scamming THEM... so if you like what you see there, go ahead & make the purchase knowing that your credit card details will be safe.