I personally like helmet lights
in addition to those bike mounted. As to their legality, to quote
'We are Cycling' "Helmet lights can be useful but don't meet legal requirements when used alone".
I use a set of
Trek Ion 100 R and Flare R City Leds, fitted onto my cycle helmet in winter and in the summer I remove them from the helmet and use them 'on the bike' as small neat daylight running lights.
Both attach to the helmet with
mounts designed to detach should you have an impact, I personally wouldn't recommend a light and bracket that wouldn't do that, I'd be worried that in the event of an impact they could pierce the helmet and do my head some damage. Although mine is a Bontrager helmet with compatible mounts and lights I do think it's advisable whatever combination you choose that the light and bracket does detach if you have an impact. I have shown the helmet brackets with lights attached both on and off the helmet, the two brackets shown separately come with the lights for attachment to the bike
My commute has a lot of side road junctions, I like that I can attract more attention with the front light as I naturally look at those cars that may pop out of a side road into my path. The front I have on 'constant', I am always in fear if it is 'flashing' a car driver may interpret that as me signalling that it's ok for them to exit the junction; the rear I am happy to have on 'flash' mode.
As I emphasised above the helmet lights are in addition to those bike mounted. I do like more than one for my busy urban commute, in addition to those helmet lights, on the rear of the bike I have an LED under the saddle, one on the mudguard and another fettled onto the frame mudguard eye. With the rear helmet led elevated above those three it does give quite a wide spread. On the front I have a Schmidt hub with LED as well as an old 900 lumens Exposure Toro LED. The below taken last night, note the visible beam of the front is the 'Exposure' led only as the bike was stationary so the Dynamo led was not contributing. That Toro has a variety of brightness levels, in that picture it's backed off to the lowest available as I ride on well lit roads, I only use the full 900 lumens when they are unlit;
the current version version 3800!