I've just sat down with the PX4D and set up my clean preset. It's a great device - really quiet when set up properly (all the presets are rubbish) and it has some great sounding effects, especially the delays and chosussessess.
The built in drum patterns are fun, too..
I looked at the Zoom B1on too, it looks great - I'm not such a fa of the floor box format, though. I'm sure it's an excellent box.