The first batches of PDFs they made (most of the Forgotten Realms things, plus Dark Sun, Dragonlance, and Al-Qadim) are excellent quality. A bunch of FR PDFs can also be found for free at http://www.wizards.com/default.asp?x=dnd/dnd/downloads, take a look at those to see the quality. These PDFs are basically remade from scratch, taking the OCRed text and putting it through layout to make a file that looks pretty much exactly like the original book.
However, doing that takes a lot of time, so someone decided to use a faster process. That's why later things are basically scanned images, IIRC with OCRed text "behind" the pictures, so you can still search them.
Be sure to read the comments of others who've downloaded the product, as some are incomplete. B2 Keep on the Borderlands was missing the wilderness and keep maps. GAZ7 The Northern Reaches was missing the all the maps. And so on.