I got my Jet this past week, and it is NOT an exact take-off on the older REI UL 30. It uses a very light plastic framesheet. It is designed to be used as a lightweight daypack, though it could easily be used as an extended trip pack by a UL'er. Its one size is too short to provide weight distribution to my hips. (I'm guessing this is true for any one with a torso length over maybe 18 inches.) However, with a UL load it is comfortable even without load transfer. By removing the the framesheet you can drop the weight from 22 ounces to about 20 (by my scales). The material is tough, the hipbelt pockets are nice. It carries well, so long as loads stay in the UL range. My only real complaint is the lack of a hydration sleeve. I may engineer a way to hang my ULA catalyst sleeve in the Jet. At $55, it is a great bargain, IMO.
As for the UL 45, they are no longer made and are available on clearance. It is a solid pack for loads under 25 pounds. Some customers who buchwhack, sit on their packs, or drop them instead of lowering them to the ground have had some issues with durability. Some have also complained that the sliding points of the shoulder straps (they are one continuous piece running through loops at the hip belt) are awkward. Others, such as my girlfriend, really like them. I have heard some customers complain they did not like the way the lid attaches at only point, fearing it will slide one way or the other with a light load. However, the internal compression system mostly takes care of that. Again, for under $60, it is a real bargain.
If you can't get the clearance UL 45 in your size, look at the Quick UL 45 from REI, this year's replacement for last year's ul 45. It uses the much tougher fabric of the Jet, a new aluminum frame tha minimizes weight gain over last year's pack, and a handsome price of $99.
These are not the cream of the UL crop, but for a hiker looking to lighten his pack weight considerably from traditional wieghts, they are a very cost-effective means to that end.