I tried it out and found the iPad app underwhelming. Not having page numbers or an easy way to flip through is making it really hard for me to use. Now granted, I like paper books, so I'm biased. But I find a PDF way easier to use than this!
I have an account through work, but I don't think I'd pay for one if it were my own coin.
Like yourself I'm biased towards paper books, and like Bear I'm completely comfortable reading fiction on a Kindle. I've only used Safari Books through a desktop browser as that's the only way I can access it just due to the way it's set up. I don't like to read entire books from a PDF but they are handy for cut'n'pasting code snips or text passages.
The perfect setup is a paper book and a PDF. I'll read the paper book first then use the PDF for reference when the paper book isn't to hand.
The problem with technology books is that the instant they are published, they are already out of date. It really doesn't matter whether the format is dead tree, online, or ebook.
One thing that's always been good about O'Reilly books is that once you've bought the book, they keep the online edition updated (at least to a degree). And I did find it convenient to occasionally employ Safari when I had a corporate account. For that matter, I still occasionally grab some useful info when Google brings up a Safari demo page.
I've always preferred to surf the edge of technology, though, so for most day-to-day documents, I'm going straight to the project website. An independently-written book may serve as an introduction - and one reason I really regret the demise of dead-tree magazines is that sometimes a pre-introduction is even better - but the final authority comes from the project itself and I'm happy to say that most quality products have at least one literate person on-board (who may, in fact author one of the published books).
I do my light reading on portable devices. For several years now, that's been a 7-inch tablet. I don't find that size screen to be good for technical docs, however. I'd rather have paper. And I've always found reading long-term on a computer monitor to be hard on the eyes.
I got a 10-inch tablet for XMas and that's big enough to read documents scaled for US Letter/A4, which is what most PDFs are. In fact, I just uploaded one of my own technical documents yesterday, complete with an embedded circuit schematic. I haven't made up my mind yet about how well that would be for a day-to-day tech reading solution.
An IDE is no substitute for an Intelligent Developer.
I have used safari but I didn't find it great. I am a fan of kindle and PDF as well. Paper backs are beautiful but they take up a lot of space and they get crumpled if carried during a travel, at least mine does. Using PDF is very convenient and simple. I love reading books like that. You could also try Kindle, great to carry during a travel.