ULA's CDT pack is remarkably lightweight, I took one with me for a few days on the Cleveland Way. with a stripped weight of just 580 grams its a very lightweight 50Litre pack that's nevertheless got a very good hipbelt, well padded shoulder straps and is very tough there are no compromises made anywhere on strength and durability, except perhaps for the main stuff pocket on the outside of the pack, which is made of mesh.
The bits I left at home were an internal mesh pouch which would probably be useful for some people, I prefer to pack my gear in smaller pouches and drybags anyway so I don't have the need for it. The padded backpanel doesn't add anything to the comfort of the pack so long as you pack it in such a way that there's nothing hard poking into your back, so I took this out to save a few grams. I also stripped out the internal bladder pouch and the removable straps. I could have gone further and removed excess straps and bungees that I would never use but I didn't do this because these would be permanent changes.
The quality and size of the hipbelt is such that you can carry a good proportion of the load on the hips, although a frameless pack needs thoughtful packing to ensure the packed kit gives the whole pack some structure. I was carrying about 20lbs and it was perfectly comfortable for most of the day, towards the end though my shoulders ached a little bit, so I can tell that 20lbs is around the limit for this and similar packs, for me anyway. Having said that, for multi-day trips so long as your base weight is a lot less than 20lbs then even if you carry 25lbs for the first few days it still works as a lightweight backpacker.
The top closure of the pack is a simple drawstring, this perhaps leaves the top a bit vulnerable to rain, however, if its not too full you can roll it like a drybag and secure it with the strap that goes over the top from front to back. The hip belt pockets are a decent size, they are square, easy to use and are big enough for the essentials like compass, light and bits of trail food.
The large open pockets on either side of the sack are also very useful, I could just about reach them while wearing the pack, so I could access a water bottle for example when on the move. The shockcord fastener really works well.
Finally the large mesh stuff pocket is ideal for stuffing a wet tent or other gear - I used it for my trusty sitmat, hat and gloves.
The CDT is a great choice for ultralight backpacking because despite its low weight its tough enough to last for long treks and it has the features that appeal to backpackers without extraneous features that many don't want or need.