For BPL's Wilderness Trekking III Program (a wintry course), we searched high and low for packs that would carry big, light loads. I think at least six of the ten people we had on the Fall 2008 program used Pinnacles.
It's the pack I chose as well.
For its intended purpose, I honestly cannot find a better pack, and I've now had the chance to use it on a number of winter trips.
My favorite feature is a big, wide, thinly padded hip belt that tucks into hip wings that carries the load against your lumbar very well. This feature should be a standard by which other frameless packs are measured.
My gripes with the pack reflect the same as Doug's: a more abrasion-resistant bottom fabric (mine is showing a bit of wear from abrasion); and better sewing quality (I too have reinforced straps on mine with bartacking).
Fix these two things and if I was the author, I'd have given the pack a "Highly Recommended" rating.