The iPad's weight is not a deterrent. If you're used to reading hard cover books, the weight is the same and the same ergonomics apply.
When using my iPad lying down, I'm rarely holding it up - same as a book. My iPad is either on the bed/sofa if I'm on my side/front or on my belly or legs if I'm on my back. When I'm sitting at a desk, my iPad is propped up using the Apple case at a 20% (?) angle. This angle is perfect for reading at the breakfast table or sitting on the desk next to my computer. In the first week of owning my iPad, I read while the iPad was flat on the table. That was ok, but now I've gotten so used to the 20% angle, that I use this all the time.
The iPad has changed how I work now, too. I use my iPad for all my email, and for my 'quick surfing' (like looking up the definition of a word, or syntax to a coding issue, or reading a wikipedia entry). It's also used to kill time while downloading, installing or compiling applications. The iPad is the best form factor yet for Scrabble!