I don't think that the ideal pocket sharpener yet exists. I've worn out several of those cheap Gerber diamond rods, and in fact once mailed one to Matthew Rappaport just so he could see what one looks like after a few months of daily usage on the job. It was so smooth as to be of use only as a fine touch-up steel after relatively little, but constant use. Ditto for EZ Lap and DMT. Although to be honest I think the Gerber wore out the quickest of the lot.
I've also tried the EZ Lap diamond dust on a plate jobbie that looks like a credit card. It's got more surface area, and doesn't lose it's dust as quickly as the rods, but it's also more limited in terms of what it can do with various blade shapes.
The Spyderco Doublestuff pocket hone is about the right idea but the ceramics are far too smooth to expediently remove dings and burrs gotten from heavy usage. The various pocket steels such as those by Schrade suffer from the same problem. They're great if all you want to do is touch up an edge in hunting camp, but if you really need to restore an edge to heavily used, dinged and gouged blades, they simply aren't up to the task.
There's a lot to be said for carrying a small mill bastard file, but unfortunately they are heavy, bulky and clumsy, and not particularly effective on some of the newer harder alloys. Another too bulky, but quite effective hone is the round two grit ax hones that look like a hockey puck. They're too heavy and bulky for most BP type adventures, but having one with you in the vehicle can be a godsend for heavily used blades.
Unfortunately, that rundown leaves little but the lowly Norton Crystolon medium or coarse grit pocket hone. The plus side to that option is that they are cheap and readily available , and can very easily be accomodated by sewing a small pouch on the exterior of the knife sheath. Their downside is that they can be somewhat brittle. Have they ever made a combo pocket hone? Medium on one side and coarse on the other like on some of their bigger bench hones?
All in all I think the answer lies in better, coaser, smaller ceramics, but the mfrs have not yet produced anything that really addresses this need. (hint, hint)