It's devastatingly effective on the upper Lea, so much so that I now rarely use the method as it's made Barbel fishing a bit boring.
If I was trying for Barbel elsewhere it would be the method I'd use and feel very confident of catching any Barbel present.
I use a large cage feeder with 4 and 6mm Halibut pellets. Dampen down some for about an hour and use these to plug each end of the feeder, light pressure , filling up with dry pellets. Band or hair rig the hookbait on a suitable sized book. I've used anything from 6mm on a 14 up to 16mm on a 2..
Cast out and wait for the three foot twitch. Re fill the feeder at intervals to suit the conditions.
I usually touch ledger with braid mainline and can feel if there's any fish around the feeder.
Edit
Shortish nylon hooklink and as with any method works well in darkness and coloured water.