I finally caved and bought an iPad last week and I am already preferring to read technical books on it over the 650: mainly this is because the iPad displays them with color, and I don't think we'll be seeing color eInk in consumer devices for some time.
The larger screen on the iPad makes reading content with tables and figures a joy, but the 650 is a nice compact size for travelling and for bedtime reading (I'm typically reading fiction at these times, so no need for the large color screen, and eInk is kinder on the eyes in a semi-dark room).
(And yes, I do feel a bit sheepish about acquiring two new reading devices within the space of a month
)