For those of use that use the iRex DR 1000S as a paper replacement and need to scribble on PDFs, the first serious competition appears to be the
enTourage eDGe, supposed to come on February.
Details and a demonstration are in this
report, in particular the second video. There's also an impressive
demo on the company's web site.
Note the user replacable battery

and the ability to merge scribbles in the device

.