I've got one of the spring creek yokes and used it to portage my SR17 during last year's trip.

Advantages:
1.  Very light
2.  Comfortable seat
3.  Easily adjustable for width during portages
4.  Quick to flip from seat to portage yoke

Disadvantages:
1.  If you fall while on the portage, your neck is in a very unsafe spot trapped inside of the yoke.  I would not let my kids carry the canoe due to the safety concern
2.  Put quite a bit of stress on a small area.  My boy goes around 150 lbs and I'm starting to see some deformation in the gunnel's metal frame after using this yoke for one week.

I also have one of the old town snap-in plastic seats that I used 2 years ago.   It also works nicely.  I used a small pair of c-clamps to lock in into place so that it would not fall out during portages.

Dave