As a happy Sony user, let me explain the limited zoom thing. Not really their fault: since PDFs are typically designed for turning into paper, they have a lot of white space around the text, so often if they display it full screen the text gets too small.
(it does show the PDF very well, BTW, so if it doesn't have big margins you're fine. And see below.)
Now, they can't easily zoom in to cut out the margins because that needs horsepower the devices don't have, to figure out where the margins are (especially with page numbers).
So what they do instead is, if you hit the zoom button while in a PDF, is extract the text from the PDF and display that instead. Which works, but they can't reflow it easily, again because PDF is designed for text.
(This is below: There's a third alternative in Calibre (open source software for converting/loading books on the device, ugly as sin but a million times better than what gets bundled with the reader) which is to use the PC's horsepower to trim off the margins before syncing it. Calibre plays nice with Stanza on iPhone too, so I reckon geeks switching to iPad from an existing ebook reader are going to keep using it no matter how fancy iBookStore is!)
So where I have a choice, I always get books in EPub, which refactors and flows and zooms beautifully, but just dropping a PDF file onto the SD card in my PRS-505 works. It's not as nice, but it works.