I bought a M 21 a couple of months ago which is my "back up" ride to work bike and fitted the same LED head and stop/tail bulbs as I have in the Ariel. I winced (once again) at the price but.......... Coming home from work is always after midnight riding through Epping Forest, well the roads through the forest anyway, come rain or shine. The headlight (6V) is nice and bright both on main and dip however how far it lights up the road is a bit amusing!
I can see well for a usable distance but then the light beam seems to run out of energy, probably half the usable distance of the 12 V halogen in my Suzuki. I tend to stick to around 50-55 on the old girl and the light is fine, no chance of overtaking it. So not perfect but in my opinion worth the cost, especially for being seen on wet dark days / evenings and having a reasonable headlight / stop, tail light for very little energy.
'53 Bantam, '55 Huntmaster, '61 Bantam, '79 GS 850, 2004 Burgman 650, 2002 Burgman 400