As a former iliad owner, i'll give you my experiences.
Size. I found the iliad to be a good size forr reading PDFs, but a bit too large for me to tote around to a restaurant or coffee shop. It's just a bit on the clunky side.
Annotations. I have fairly messy handwriting and write pretty quickly. The iliad's notetaking ability was almost completely useless to me. Most of the time i had scribbles that hopefully jogged a memory about what I was trying to write. I found a tablet pc to be far more effective for meeting notes.
Software. The mobireader software is ok if you're just reading fiction. For any kind of reference work it starts to show it's weaknesses fairly quickly. I'd recommend FBreader right away.
I have a 17 yo son and 13 yo daughter. I don't think the perfect student eink machine has been made yet. I think it's going to take a bit of give and take on both manufacturers and publishers parts to put a student reader together.
From a business perspective, the iliad left a lot to be desired. The best part about it for me was FBreader's CHM support, but that was naturally something irex didn't support out of the box. I thought the build quality was closer to a prototype machine rather than a consumer device.
I wish you well in your search, but in my estimation, textbooks aren't a good fit for eink readers just yet.