Tried one, but it wasn't for me. Still like my waist pack... an upgrade from my fishing vest days. Several of my guide friends use them, but that makes sense given all the tippet management, fly changing, etc. they do as part of their job.
i love using a lanyard. if im just running down to the river durring lunch or jumping from spot to spot on a few highway sections, i can have my lanyard ready with my tippet, nipper, and hemostat, and just put a puck of flys in my pocket. i dont really like fish with a pack if i dont have to. if im spendin a few hours, ill wear a pack, but ill still wear my lanyard and have my most used items right there... less digging around. theres enough weight to not flop around, but not so much that i feel it on my neck. mine is homemade- paracord style- like $2.
I use a lanyard ( don't recall the name) when I'm stream fishing for trout. On it I keep a Leatherman Micra (the one with scissors), bottle of Loon Top Ride,bottle of Loon Aquel, a couple Thingamabobbers, 5 or so tippet spools, and a leader straightener I never use. With a lanyard I wear a Simms Hip Pack. If you keep it clipped to your shirt and not to heavy, it shouldn't really get in the way. When I'm streamer fishing or chasing steel, I use a piece of paracord to hold my Leatherman Micra around my neck and tucked down my jacket and all my other terminal tackle is in my wader pocket.