Spyderco Sharpmaker is my favorite for maintaining a sharp edge or getting an already pretty sharp edge, sharper (like fixing a factory edge a bit). However, it isn't good for having to reprofile a blade. It would take a long time to do. For that the Edge Pro is better, but much more expensive. Both systems aren't really that portable though, but they are pretty fool-proof, which makes them great for a "newbie."

For field sharpening I love the DMT diamond stones. Especially their double sided diafold sharpeners. They work great for sharpening and maintaining an edge. The only problem is you have to be able to free hand sharpen (most importantly keeping an angle), which takes practice. I have a Blue/Red double sided diafold and a green mini-sharp. The blue/red gives me a razor sharp edge and the mini-green works like a strop to keep the edge maintained to shaving sharp.

The blue diamond sharpener in my bag is from walmart. It will sharpen both plain and serrated edges and it takes up very little space (as much as a pen). It isn't the best sharpener, but it works enough to put a usable edge back on a knife. It's also pretty cheap, and it's very versatile. Like the DMT stones though, you need to be practiced in freehand sharpening.