From everything I've read about the iRexes, it sounds like they'd be the only viable e-Ink devices for doing the sort of things you'd want to do. When I tried to search a bit on battery life, it looks like it's less than ten hours.
I'd caution against forcing yourself to adapt too much to accommodate the weaknesses of an eInk device. The Iliad will have screen lag (like any eInk display) and will not have color. To me, those would be some pretty serious weaknesses. You can't really "highlight" with black ink, can you? Strikethrough, sure...or underline, but not highlight.
If a tablet PC is a far superior tool, you might want to consider not fixating too much on eInk. Technology changes so fast, I'd avoid buying tools that are subpar at performing any of your required tasks.
When I read this, I almost held off on buying an eBook reader altogether...
The CEO of that company is a pretty well-known screen person with lots of cred (was key on the OLPC project). So this isn't just some penny in the wishing fountain--she has a good chance of delivering.
Then I realized, you know what? Even if they released a Pixel Qi device tomorrow that was amazing, the Sony 505 would still be an excellent, simple eBook reader...because even if some fancy Pixel Qi device came out...if all I wanted to do was read text on a page, I wager it'd look the same. Whereas, if they released a Pixel Qi tablet PC--where it was a tablet PC that could do everything a laptop could do but had a very eye-friendly display--suddenly the Iliad is crappy in comparison with its laggy tablet abilities.
Anyway, that's just my perspective. You'll have to figure out your needs and priorities on your own