I have run pairs of 606's two times on my 600, also K270's twice, t63's twice and an MT21 front one time. The MT21 was the fastest wearing by far and seemed to take off every other set of knobs as it wore. The others wore out at about the same rate, 3500-4500 miles.
I do not like the 606 front, but good on the rear. The K270's seemed to have too much wiggle on the road. The T63's were the roundest and best feeling of all on the slab, and worked good on dirt too. Lately I have tried Shinko 700's. The rear lacks traction on dirt, but feels good on the street. The 700 front is by far my most favorite front tire yet. feels great in the dirt and gravel roads and so far has worn 4000 miles and is not yet half gone.
Replaced rear with Shinko 244 to get better (hopefully) dirt traction, and turned the front 700 around the other direction to wear evenly. I generally rotate the front that way, half way through. The 606 was too directional to do that though, main reason I don't like them on front.