Hey Smudge, I have a Spyderco Sharpmaker sharpener which is a very similar concept to the Warthog - you hold the knife in your hand and push it down and pull back, with two vertical rods set at the edge angle etc. It looks to be a bit more 'free hand' than the Warthog - doesn't have the criss-cross effect.
I think it does a pretty decent job for my filleting requirements, and packs down easily to take on holiday.
There are some excellent comparisons vids on YouTube, and the Warthog scores well. One did a very detailed analysis of not only how sharp the knives get with each system, but how long they stay sharp for after sharpening with each one. Surprisingly, it does make a difference. I would have thought once a knife is sharp to the same level it'll deteriorate at the same speed, but actually that wasn't the case due to some reason about the shape of the edge the sharpening system creates.
I've seen plenty of instructional vids on how to use a stone, but setting the angle the same every stroke sounds a lot easier than it is, in my opinion. Certainly if you're not doing it daily.