does it remember where you were: yes
Can you skip pages: yes
atm, the best pdf reader by far is the iLiad, yes. This may or may not change Q4 2008
is file size an issue: not generally.. i can easily read scanned 100mb+ books
Rendering complex pages: this is different from size.
Tables, pictures, diagrammes take more time to render than text, and it is possible that specific pages are too complex for the iLiad to render (it after only has so much CPU/RAM to work with)
differences between BE and normal: no others that i'm aware of.
Wifi: if you've got a router, is indeed useful. if you don't (atm, ad hoc networks are nearly impossible to set up)
that said, it's also handy to have a card reader to fill your CF card up with, as the internal reader/writer is fairly slow.
battery use: the screen does not use additional power after rendering (unless you have the stylus out and near the screen), but the CPU does run at 'idle' (it can't standby) which does use power)