8:31 p.m. on September 17, 2012 (EDT)
If you'll be carrying 25 pounds or less I'd recomend the Go Lite Jam 70 liter -
Not that I have all that much experience with all sorts of backpacks mind you - But this summer I also decided to "get light!"
I've been carrying a 75 liter karrimor jaguar since the mid 80's and it really was time for me to get a new pack anyway. My wife has an REI Morningstar that works just fine ( although if it were me the top flap/pocket would drive me nuts...) but it is on the heavy side. So after much looking about I found the 70 liter Jam on sale early this summer and got two.
These packs are huge, especially for ultralight rigs. One has to be carefull not to overload them, but they are great for bulky items. The only reason I got the 70 liter packs was because they were out of everything else, and Go Lite insisted the compression straps used on the 70 liter pack would allow more compact loads to be carried as easily as the smaller packs do.
Oh, that's also one of the six-ounce Go-Lite umbrellas my wife is using while hiking along in the rain -
Having tried the umbrella thing now I'd take one along on any thru-hike.

The 70 liter Jam is a huge pack -

This shot of me scooping water from a tiny seep also happens to show the suspension system of the Jam.

It's pretty comfy, but note that it isn't adjustable for height. You got length adjustments on the hip belt and shoulder straps, and that's all folks! So make sure the pack is sized for you. I got a "long" one after measuring my torso, but I think I should have gotten the next size down.
I pull the hip belt just as tight as it can go - I have a 30 inch waist, am 5'7", and 140 pounds.
The pack weighs just under two pounds, but I gather dedicated ultralighters can easily carve a full pound off the weight of one of these, starting with the hip belt. For me, the jurry is still out on the hip belt. On a recent trip I experimented with using the hip belt and not using it - And I found the hip belt does take weight off your shoulders even without much of a fancy frame in the pack
The reason our packs looked so stuffed for what was just a weekend trip is because both our packs have a full sized Wal-Mart foam sleeping pad in them. Coil the pad, put it into the pack and uncoil it.
Now put everything you want to tote along inside the circle of foam pad.
These pads are light, cheap, and warm but they certainly are bulky. But it sure did stiffen the pack. We could easily do away with the backboard the Goliote comes with using this packing method.
The 70 liter pack has compression straps along the sides, but it also has a hook and loop arrangment along the bottom to pinch the outside and inside bottom of the pack tightly together, really reducing the volume. Great for long trips where you are carrying allot of food at first, but several days down the trail need to compress the bottom of the pack to keep your remaining weight where it should be.
The packs are well made of good materials and are simply designed. I like that, and have always prefered the "Bloody great sack" ( as Colin Fletcher wrote ) sort of backpack.
I sorta do wonder about how well the mesh side pockets will hold up in the long run, but others seem to say they do alright.
The Jam has a big conventional zippered pocket at the back, unlike the big mesh pocket of the Gossomar Gear G4 and the original Ray Jardine backpack. I have mixed feelings about this. On one hand I have found that pocket darn handy - Maybe to handy, and have wound up cramming in more odds and ends than I really need to -
And I like the idea of being able to stuff a wet tent fly or tarp into the big back mess pocket of the original ultralight packs, so I don't have to stuff wet gear inside the main pack, with my dry clothes and food.
I didn't go with the G4 because the Jam has that hip belt. But if you can keep the weight to twenty pounds or less, I'd think about the G4.
I reckon I could do 30 pounds with the Jam if i had to carry lots of food, but it's best at 25 and down. I reckon one might push the G4 to 25 pounds, but I think that it's probably best below twenty.
Kinda depends upon what yer base weight looks like it will be.