Trying to say things that I haven't seen said rather than just "THIS".
So, I bought my first nice knives this year and, being the obsessive kinda person I am, that lead to me having to know everything I could about sharpening. A couple thoughts:
I think stones/diamond plates are the best, all-around tools. The downside to stones is that they wear and need to be leveled periodically or your sharpening efficiency will go down. I wound up going with small diamond plates... More expensive to start, but easily carried and efficient.
(edit: If you get comfortable with using stones, you have the benefit of training your hand to find the blade angle and maintain it - as opposed to depending on a sharpening system to maintain the angle for you. That means that, in the event you really had to sharpen a knife in the field in an emergency, you could likely do so sufficiently to cut what needed cutting using two flat rocks - one more coarse, the other more fine - that you could find near a river or something.)
The other aspect to sharpening that I haven't seen addressed here is blade geometry/grind. Make sure you know the real shape of the blade from the bevel to the cutting edge and, if the knife has one, at the cutting (aka secondary) bevel that actually runs at the sharp edge. If you don't know approach sharpening the knife in a way fitting for that blade shape, you'll find yourself having to work harder to get things cleaned up and you'll not have a lot of cutting efficiency.
Last thing, blade maintenance. In addition to whatever you get to sharpen/rework the blade, get or make something that will allow you to easily touch-up the edge after every use. For example, a small ceramic rod or leather strop with polishing compound can be used to 'polish up' the edge and take out small rolls quickly. This keeps the razor sharp and keeps you from having to use the stones (which remove more metal) as often.
Hope that helps and adds to the discussion.
edit: FWIW, here's the best article series I found on knife sharpening in my internet travels. Hope it helps. http://forums.egullet.org/topic/26036-knife-maintenance-and-sharpening/