I use a Packa, and works great...
I have a Kelty pack of about 4000, and my Packa fits it fine. I used my Packa all the way through my section hikes in GA this past summer, and intend to carry it on my upcoming thruhike attempt next month. I still wear a nylon windbreaker top (longsleeves) underneath for sun/bugs/brush in all temps, and may wear snow pants when there is white stuff on the ground.
BTW, I had a phone discussion yesterday with one of the pros at Mountain Crossings (outfitter @ Neel Gap; great people) when I called to check on my final pre-thru gear order. He told me that Frogg Togg pants tend to blow out so frequently that they lost interest in carrying them any more. They offer some other brand of (heavier) rain/snow pants instead. I keep telling MC to start carrying the Packa; Cedar Tree, have you ever talked with them?
Yet another Packa on Ebay