It all depends on your values and your budget. If your wallet is looking thin, you might want to think carefully. I personally don't think $20 is that much money. Pretty sure I can save an extra $20 any given month. That's basically spending one less dollar on food every day for 20 out of 30 days. If your budget is really that tight or you have big debts, then please don't. Fan translation groups will continue to exist.
Putting costs aside, what you'd be getting is really good and I have to second Romana's recommendation. I Hate You is one of my favorite works to come out of the subscription so far. Also if you want to see a sample of a FAKKU translation, there's half a color chapter out on the FAKKU Tumblr,
check it out here. If you're still unsure then you can wait and see for yourself. There are plans to have some free content out after fan translated works are removed for good.
I recommend just trying it for a month. You're free to cancel whenever you like, and it'll all be there if you come back later. With all the content we have now it should be somewhat representative of what's to come as well. I think there's great selection right now and a fairly diverse spread of tags.
And as for your other concerns...
>Access to magazines
Subscription is all magazine content, which means you have access to every magazine release, current or past.
>Access to books
Sorry, but those aren't included. The books are separate from subscription works, and you'd have to pay for access to those individually. Buying a book gets you permanent access to that book though.
>Security
Not sure. I think FAKKU uses a credit card payment processor but I haven't looked into it. Maybe @jacob or one of the developers can tell you. I paid with Bitcoin which was handled by Coinbase (BTC payments will be handled by Bitpay in the near future).