I just purchased a MLD Zip myself a couple of weeks ago, and can't wait to try it out when it arrives in mid-March. I emailed Ron obsessively for weeks before finally deciding on the original Zip, over the Super Prophet, and Super Zip. Ron told me in the emails that a bear canister should fit vertically inside of the Zip, no problem. This is a concern for me as well since I'll be using the Zip on my PCT thru hike this year.
Ultimately I chose the Zip because it had all features I wanted (minus hip belt pockets that I'll add myself), really lightweight with solid material, it's NOT a gossamer gear pack with shady stitching, has a huge back mesh pocket which I fell in love with on my ULA Catalyst, and has tons of configurable compression options. Plus, the MLD products are a joy to look at, and are always reinforced with bar tacks wherever they are needed. I mean...the website lists over 50 bartacks. I wonder how many Gossamer Gear use? The shoulder straps are also thicker than those on Golite packs, and I've read great reviews about the comfort of the MLD suspension. Of course ULA packs ride great as well. The only problem I had with my ULA Catalyst last year on the AT was that I always wished that it was a bit taller of a pack. The Zip is a bit taller and slimmer, which I think will keep things nice and snug against the bag, not pull downward on the shoulders, and eliminate the need for load-lifters which I always needed tightened on my Catalyst.
The other packs I was considering were the ULA Conduit, GG Miniposa, and Golite Jam2. I've never used a frameless pack before, but I'm confident that I've made the right decision in chosing the best one.