I have the same Arcteryx waist pack and I think is great but I can't seem to get it right on me. I love them for biking but for hiking I prefer a good old backpack. My biggest beef with waist packs is that I carry on my belt always a leatherman and a glove pouch. Them two when combained with waist pack belt make for very uncomfortable combo.

I want to use them more since a lot of times I have enough gear for waist pack and dont need so much space that backpack offers but I opt for the latter becaude of a more comfortable fit.