I have used both the IRex and the Sony Reader.
After using the IRex for a month, and always having to wait almost a minute just to read after turning it on, (If you don't turn it off, the battery continues to go down), I then accidently dropped it and broke the screen. ($343 to get it fixed, will not do). I did like the flip bar, but battery life sucked. (Around 8 hours). I like the ability to have directories to sort your books.
Got hold of a Sony, and it only takes 3 seconds to turn it back on, and it returns to where you left off. (With the IRex, you have to go through the menus to reload your book.) I have used the reader for days, and the battery icon still hasn't changed from full charge. The screen is a little smaller, but at least you can zoom some. The weight of the reader is good. (I didn't notice the weight of the IRex, until I held a Sony, much nicer). I wish it had directories to sort you books, but if you have the books on the internal memory, you can put them into collections. I have a few of them, current reading, work, religious, etc.
In the end, the Sony is the better choice: half the price, extreme battery life.