Has anyone every installed one of these on a GS500?   I saw this on another motorcycle... very effect way of catching the attending of someone behind you.

Some articles I've read say they are annoying and becomes less effective at stops but I'm thinking if it flashed several times then went solid red that would be equally effective.

I installed a Backoff XP and it works great.  Installation was a snap.  There are two flash options.  I chose the flash 5 times then solid red.  The XP works with LEDs, which I installed to replace the original incandescant bulbs.  I think the LEDs work better with a brake light flasher since they are instant on and off.  And for those LED critics - the bulbs I bought are at least as bright as the OEM bulbs.

I pulse my brake by manually hitting the brake a couple times - and only when I see someone behind me. I do that in the car, too.
Too much flashing gets ignored and can cause a hazard due to eye-fixation.
How often to you watch a squid on a bicycle with flashing lights? Not often - you look away because it is sooooooo annoying.
Loud stereos? You turn away. Same with loud bikes - people don't like them so they don't watch them but turn away.
A couple flashes may be OK but not continuous flashing (which could also be illegal as emergency vehicles use that)

I bought one from here. It flashes about four times then goes solid red, and if you tap your brakes repeatedly, then it flashes less before going solid.