I know there is a ton of info out there already on these, but I thought I'd add some Gasser specific content. My bike is a 2012.
Here is my stock light output at about 30ft, not even enough for my camera to focus:
Here is what came in the nicely packaged box I bought, this silver dohicky is the rectifier:
You can see the various base plates.
Behind the stock mask:
The baseplate I needed was a bit deformed but a minute with some snips and a file and it was cherry:
Here it is installed, getting the retainer spring on was a bit of pain but slow and steady did it.
I butchered the rubber boot trying to trim it to fit and made a mess of things. You could probably get away without trimming but installing it in the housing would be a real pain.
Waaaaay brighter and lights from the starter turning the motor instantly. There is no flickering above idle only when the engine is warming and running rough below normal idle speed (It's below freezing out right now when I was testing)
The unit is quite a bit larger than stock and mine rubbed on the head stock, I put some foam behind the mount straps to space it out but will try to come up with something more permanent.
Overall I'm pleased with the service, shipping time, and product. My bike is only wired for a low beam so I'm not sure if I'm missing out on any brightness. I'll have to do some research and see if I can just jump the high from the low and run both circuits simultaneously or if I need to do some rewiring.
I get caught out after dark enough times due to broken bikes, over-ambitious ride plans, or just plain sillyness that I wanted something dependable that ran right off the motor and didn't depend on the battery to work. I think this foots the bill nicely.
If I could get a some more light out of it by utilizing the high beam then I'd say it's bright enough for a decent trail pace. It's no squadron but for <$100 and ease of install it is a bargain for sure. For serious night riding I'm going to look into also running an additional light off the battery but expect that to be a full loss system.