I think it's a fine line between useful and annoying.
I just bought and installed the Signal dynamics modulator, and I found that my headlight was turning completely on and off, not the dim/on it is supposed to. The full (or close to) off/on flash is definitely way annoying, even to me seeing the reflection off other cars. That said, the faster flashing rate on the SD module (which is dim/on due to the higher frequency) looks alot less annoying and much more obviously not me blinking at someone (as has been discussed in other threads, of people thinking you're signaling them to turn in front of you or something like that). After a couple e-mails with Signal dynamics, I found the following things:
1. The SD modulator only works with incandescents (OK, it says this in the small print...)
2. The reason for #1 is because the modulator just pulses the power to the headlights on and off completely, and it relies on the slow cooling time of the incandescent filament to prevent the bulb from dipping below the 17% lower limit from FMVSS108 (federal reg)
3. My modulator flashed at 2.76Hz. FMVSS108 states the frequency requirement as 4 +
20BPM). So the SD modulator is not actually legal
. This appears to be maybe a recent quality problem for them. The guy at SD confirmed the low frequency himself, and I measured the frequency with a Fluke DMM, so I'm 100% certain that the frequency is actually low. SD said that they don't have access to the source code of the headlight module, so they can't update it to fix the problem
4. SD is releasing an LED compatible version this summer sometime. I would assume that this module will not flip the power on/off, but more of a 100%/33% type cycle, or something like that. I don't know if their frequency circuitry will be any different, but maybe it will be
5. The switching schemes built into the SD modeule are very nice. You can switch modes with 2 honks of the horn (if you've hooked up the heads up horn wire). You can also turn the modulation off by flashing the brights for less than 1s duration. I am able to just turn off the modulation right now with the defective unit, so that is really nice. I'm pulling it off the bike tomorrow and returning it.
Anyway, just thought I'd share that interesting info on the newest headlight modulator thread. I think I might wait for the LED compatible module this summer, as I can't find any modules with the coll features of the SD one for less or equal price.