I'm with Nick. Having a framed pack with the stays that fit the curvature of your spine makes for a much more comfortable trip, even when costing an extra 1-2 lbs. The reason is 1) the pack moves with you, 2) you can get proper weight transfer and, 3) you can have proper hiking posture. Sure, a framed pack bumps me out of the SUL weight class, but do I really care about that, when I'm more comfortable when I'm actually outside? Not at all. The purpose of going ultralight is 1) minimalism
and 2) comfort. A framed pack doesn't impact 1), and greatly improves 2) (for me), despite being in good shape and an active 30 years young. My base weight with a 20oz pack is between 6 and 7lbs, and most trips are sub-15lb. Even at these weights, the difference is substantial.
Even if you pack a frameless pack correctly for proper weight transfer to the hips (packed tightly), you'll have a triangular gap between the shoulder strap connection and your upper back. This makes a pack carry less stable and creates a rearward force when hiking. You can easily compensate by slouching forward, but over long miles, this isn't very comfortable. Many people will say that you can use soft goods in the pack, and punch the pack into submission (to form to your back), but this doesn't last. I found that I had to do it quite frequently - the formation wouldn't stay that way. It was a real hassle. This is because you had the pack it tight in order to get good weight transfer.
I've tried both the Burn and the Gorilla, with and without stays and sitpad.
I found the Gorilla very uncomfortable overall. The sitpad inteferes with pulling the hipbelt tight, by creating firm corners that dug into the back of my hips. The pack was infinitely more comfortable with the sitpad INSIDE the pack. The shoulder straps were too wide as was the hipbelt. IMO, they're unnecessarily wide, and are created in a 'squarish' design that doesn't work well with curves of the human body. When you add the hoop stay, I found that even though you can curve it to fit the curvature of your spine, the connection to the hipbelt is quite poor. The frame (and pack) will rotate away from the hipbelt and create the same backwards forces that a normal frameless pack will do. My opinion is that any frameless pack with a hoop stay will do this. If you want a frame to do what it does best, you have to design the pack AROUND the frame, and create an extremely strong connection to the hipbelt to avoid this backwards twist. However, I absolutely LOVED the side pockets on the Gorilla. They're very well structured and make it very convenient for water, food, a rain jacket, etc.
The Burn was fine, but it still had the backwards forces that all frameless packs tend to have. I also found the hipbelt too short to properly grip my hips. It wasn't much better than a webbing belt.
IMO, if you want to try a frameless pack, skip the Gorilla/Swift/Ohm and go all the way with something like the Murmer Hyperlite, SMD Feather, MLD Burn or ZPacks Zero/Blast. All the fancy convert-a-frame features in the world don't make up for a pack that's really a frameless pack with a frame added as a secondary thought.
IMO, frameless packs are good if you don't mind carrying most of the weight on your shoulders. Then you can pack it loose and conform it to fit your back for a good fit, and you can have proper posture when hiking. The minute you pack it tight enough for hip weight transfer, you lose the ability to conform it to your back, and you have to begin hiking in a forward leaning posture.
Note that with a framed pack, you don't have to carry all the weight on your hips. You can easily loosen the hipbelt to variably shift some (or all) of the weight onto your shoulders. I do this all the time.
I find that a good pack is like a good sleeping pad. If you find a pack that allows you to hike comfortably, screw the weight. If you find a sleeping pad that allows you to sleep comfortably, screw the weight.
EDIT: Another big problem with almost every frameless pack, is that they have hipbelt wings. There's a big difference between the comfort of hipbelt wings, and a centrally-attached hipbelt (where the attachment of hipbelt-to-pack is ~6" wide, in the center of the pack).
EDIT: You don't need a frameless pack to have it be lightweight. It's just an unfortunate side-affect of manufacturers that make framed packs. I'd love to see a cottage gear maker like SMD/GG/MLD/Zpacks design a REAL framed pack, where the frame is an integral part of the backpack (none of this hybrid crap). You can easily make them around 20oz. It's unfortunate that no one does this and you have to enter the MYOG world. I've spoken with Paul briefly about the 16oz framed pack he designed:
I plan on making one myself, soon (waiting to find some DX40-like material). I'd gladly pay $200-300 for such a backpack. Do you hear me SMD/GG/MLD/Zpacks!?!? No one makes anything like this on the market! Yes, you heard me right... absolutely NO competition AT ALL!
I even contacted Chris Zimmer at Zimmerpacks, and he didn't want to design this sort of properly framed pack. The only framed pack he would do was the traditional frameless-converted-to-framed backpack, with hipbelt wings and corner stay attachment. I was pretty disappointed.