I’m a convert. I have removed my hip belt pockets in favor of a waist pack. I use a 1 L High Tail Designs. I would like a 1.5 L, but it’s probably for the best that I don’t have as much room, it keeps me from squirreling away more than I need at my fingertips. I found that the lack of hipbelt pockets made for more comfort when using trekking poles, I didn’t have to worry about banging into the pockets.
I tried a Thrupack Summit Bum 2.5 L, and found it to ride saggy and jouncy. The High Tail has supportive “wings” on the belt that holds it flat. The Summit Bum has an exchangeable belt that slides through a tube on the back. The tube is attached to an outer pocket that is great for stashing a cell phone or snack bars, but causes the pouch to sag away from the wearer. The 2.5 L size creates weight and bulk, and invites you to cram more stuff than you really need. The High Tail is lighter, with a thin hipbelt that feels stiff, but becomes unnoticeable under the hipbelt, and doesn’t collapse and roll when sweaty. It also comes in pretty watercolor painting designs, which is a nice touch compared to all the utilitarian designs. Admittedly, I’ve been eyeballing the Hyperlite Mountain Gear Versa, which has approximately 2L capacity with a stable belt configuration, but the discipline of restraining myself to what I actually will use during the day is still good.
The other place I think waist packs are good is that if you are crossing a river, with your hipbelt unclipped, if something goes completely south, you could have at least some firestarter on your person when your backpack and shelter is floating away. A small waist pack isn’t going to be a floatation risk.