I would add another recommendation for the Fällkniven DC3 or DC4. Either model works great as a combo stone. The diamond side is high quality and lasts a long time (low-cost diamond sharpeners tend to have a pretty short service life). The ceramic side is fine enough to give the edge that smooth, shaving sharp final polish.

I own a bunch of sharpening tools, stones, jigs and power tools (including a belt grinder) but I keep coming back to the DC3 all the time - it just works really well and it's so versatile it can be used on anything from a knife to an axe or machete.