LumpyJaw

Equipped has a fairly good article on sharpeners.

http://www.equipped.org/devices13.htm

I have used the carbide type in a jam, but find that the crock type work just a fast but leave a better edge. Gatco and Gerber have a small pocket ones that can be carried in the same space as a pull thru, these aer cheap about $5 each. If you have a good edge to start with these will keep it that way in the field, but you wouldn't want to try with a really dull knife.

If you must get a pull thru, then the best is Meyerco:
http://www.amazon.com/Meyerco-Sharpen-It-with-Nylon-Sheath/dp/B0001WP41C/ref=pd_sim_dbs_sg_title_2
or
http://www.knifesite.com/Meyerco-Sharpeners-s/162.htm
I have one in my BOB along with a diamond set from DMT.

Also see :
http://www.knifecenter.com/kc_new/store_store.html?ttl=Pull%20Sharpening%20Products&srch=eqWWWCAT_1datarq%3Dsharpen%26eqWWWCAT_2datarq%3Dpull