I'm a previously long-time Microsoft Reader user (old Ipaq), who upgraded to a Sharp Zaurus and while the screen and hardware are better I actually found the newer device to be worse for reading English text. -- It is great for Japanese.
I am planning to move back to Japan early next year and in preparation for a long commute
am looking at the Sony Portable readers to ensure I get a device that is firstly a good reader and not a good PDA/ok reader.
Not yet decided but most likely going to order a PRS-505 from B&H.