I always had a spyderco ceramic double sided stone taped in it's sheath to my scabbard. Everyone who borrowed it said it worked awesome....I wouldn't know, I sharpen like NDD.
NDD, I ended up getting a Lansky knife sharpener...that's the ONLY thing I can sharpen a knife with.
I like the spyderco ceramic cuz it doesn't load up that quickly and you can use spit as alubricant. Cleaned up really fast with a toothbrush and some soap.