I've worked with pdfs for the past ten years for a large printing company.  It's a durable file format, fairly universal now, and are an excellent medium for electronic sales.

I think that as it stands now, most companies that sell pdf versions of their products have gotten the pricing right.  Products found on DriveThruRPG and RPGNow are, generally, decent pdfs and the price is reasonable for what you get.

I think adding a good set of bookmarks is easy, and should be standard for any RPG pdf.  It's like an index, and no good RPG book is without one.  Hyperlinking, while convenient, is generally less important, since links can and do change frequently over the lifetime of a website... unless the hyperlinks are updated in the pdf file when they change.

I would find it fairly outrageous for a company to offer a pdf at hardcopy book prices.  I would rather get the hardcopy over the pdf, if both are the same price, because a pdf cannot beat a book for feel and comfort value.  Curling up on the sofa with the laptop just doesn't have the same appeal as doing so with a book.  And the various e-readers out there are not yet up to snuff in my opinion., and things like pdas and cellphones make reading books aggravating due to their relatively small screens.

When pdfs came out as a viable option for buying RPGs, my RPG buying habits changed.  I tend to buy the main rulebooks (the core books for World of Darkness, the three main books for D&D 4.0, the two main books for GURPS 4e, etc etc) as hardcopy, while splatbooks, campaign books and other background and filler books are purchased as pdfs.  In this way I can get the whole collection, while saving myself a bit of money.

So, in buying a pdf, I am trading the aesthetics, comfort and convenience of a book for ease of transport, storage and printability that a pdf requires.  At high prices, I'll choose the book over the pdf (though I will buy less books), while at lower prices (50% to 75%) I'll choose the pdf over the book.

Most publisheres have the right idea when it comes to PDF's.  I have been a customer of Drivethrurpg.com since the website first came online and have spent more money there in the past years than I have spent on any other RPG products in the same time period.  The only company that I really think droped the ball on PDF products is WotC.  While they offer the entire pre-third edition TSR catolog at $5 a download they charge full price for all products for third edition and later.  Even after 3.5 came out all the 3rd edition books were still full cover price.  For the most part many PDF's are a great way to get books you wouldn't nessarily pay full price for or especially long out of print RPG's that are hard to find.