I own one. I bought it to use with my Zaurus C1000 and I haven't used it with my N800. To give you an example of what's it's capable of powering:
-It will let me use my 2GB memorex USB flash drive, which wouldn't work with the Zaurus's USB host mode when plugged in directly.
-It won't let me use a 1.8" USB hard drive. It tries to spin it up but fails .

In short, buy it if you're feeling lucky, but it really doesn't provide a lot of juice.

-John