I also previously used a Z55 and as my load lightened, I felt the need for a lighter pack to match it. I ended up going with a 2009 ULA Ohm that I picked up off of the gear swap here, and and extremely happy with it. From what I've read, the new model has an improved hipbelt (although I don't have any complaints with the one on my pack). I have carried loads up to 30 lbs in all day comfort. I will actually be overloading it probably to around 35 lbs for a 5-day trip I'm going on leaving for tomorrow, and I also plan on camping at a couple dry ridge-top campsites, which will mean hauling 3L of water at the end of my day. I will definitely report back as to how the pack fared.
My only concern with the pack is that so far, I've only used it in winter and cold spring conditions, and even then I had a super-sweaty back (and I'm a fairly mild sweater). Not that I didn't get sweaty at the contact points of the Z55, but it was less because there was a large dry area in between upper and lower sweaty spots at the shoulders and lower back, as opposed to an entire sweaty back with the Ohm. Again, I will report back as to how that affects warm-weather hiking. It's looking to be in the 70s-80s my entire hike, with only one day of possible isolated t-storms.
In terms of volume, I'd say the main pack bags are similar in size. It's a bit easier to pack the Ohm, though, because there's no aluminum frame protruding into the pack bag. Although you can't carry as many things on the outside of the pack, the side pockets are by far the best I've ever used. I have heard many others attest to this. It's the only non-daypack I've ever used that allows me to EASILY pull out and put away a water bottle. This is huge to me, because I always carried a hydration system before getting this pack, as I like to avoid taking the whole pack off just to get a drink. Couldn't do that without a hose in the Z55.