I almost bought one of these, but the support of only TXT files made me change my mind. The device is capable of being modded though, and as you say they have a PDF reader in development. There is also an ebook reading mod out there that lets you boot directly into just an ebook reader instead of the PMP OS. Still, I felt the SmartQ 7 was a better deal for overall capability and price. On the flip side, the screens on these Onda devices are really nice and the build quality looks very solid.
Onda just released a 7" version of the device as well. You can find it here:
Myself, I'll stick with my Q7 until something better for my needs comes out. That Onda is really nice though.