The problem with the harken travelers is that they move too easily. When you're traveled out and you pull in on the mainsheet, the whole stack ends up being pulled to windward.
I find that the stock system works just fine with one ball and one slider. Spray the track with silicone before sailing.
This is true, as I mentioned above. However, I didn't notice it being that much worse or different than the ball and slug Hobie traveler in that respect, and it is very nice if you opt to play the traveler for instead of the mainsheet. I don't race and I kinda prefer the Harken traveler, but as I said above, I don't think I would have tried it were it not essentially free (aside from the crossbar swap which had to happen anyway--another story). I really don't see anything wrong with the Hobie traveler design at all.
The boat my rear crossbar came off of was previously owned by a crazy Frenchman who loved to go out in super heavy air and liked to play the traveler instead of the mainsheet--which is fine, since it is pretty much what a traveler is intended for (set leach twist with the mainsheet and work the trim to apparent wind with the traveler).
I like it enough to have not bothered to change back, but there are a myriad of other things I would spend my money on for my boat before doing that swap.
Now, if only the traveler on my 30' ULDB monohull moved as easily. There I would _love_ a Harken windward sheeting traveler (but being broke sucks).