From what I have read and been told, stay away from ones with metal sharpening guides. Find one that has just ceramic and if you want a pocket one, look at the Lansky one below. LCKEY is the model and you can remove the ceramics to clean them.
If you want something better, look at Spyderco Sharpmaker for around $55 or the Lansky 4 rod sharpener for about $20.