I wore my Dakine Heli pack for a lot of the last couple seasons mainly because we usually did day trips so we try not to stop at all unless we really need to piss or something. I put a camelback in it and just enough water to last me the day because if you put too much shit definately gets heavy and throws you off balance. Its also nice if your hitting hike to terrain cuz you can just strap your board to your back. Check it out, I think it was like $75 or something...I wouldnt go any bigger than this pack tho for sure, it may look kinda big buts its really thin so its not bad at all, really light too.
I dont fully agree that you cant progress while wearing one because Ive learned everything up to 3's and switch 5's while wearing the pack and the first time I hit a 60 footer the pack cushioned my landing because I leaned too far back. (and no it wasnt the pack that made me lean too far back, i was just being a puss)
That being said I started riding without it towards the end of this season and liked the extra flexibility I had, so will probably only rock it when im doing backcountry.
Just depends what your comfortable with