I have a Sony PRS505 and I bought my iPad last Thursday.
For me, the two devices are not truely comparable. I didn't buy it as an ebook reader and have no intention of giving up my Sony. All of my reasons for purchasing the 505 are still valid and the device is still perfect for me needs.
I do have to admit that books are pretty on the iPad and I have reading plans for it, just not for it to be my exclusive ebook reader.
It all comes back down to personal preference. If I had bought my iPad to be an ebook reader, rather than a photo/video device, then I'm sure I would feel differently, but I bought it for other things and the book reader is only a bonus to me.