I'm running the Boyer Powerbox. Would have been much cheaper to wire in a cap as T120 says, but this is a neat, compact solution.
Bike starts and runs batteryless, but lights flicker at low RPM and it likes to die at idle. (I run all-DC wiring.) It's a great backup to have, and has enabled me to get home one day when I blew a fuse, but I still run a smaller dry cell battery because there's no real drawback to doing so.
Edit: Bike's not that hard to start, either--haven't much noticed a difference with or without battery, hot or cold. Spark seems to be spark seems to be spark.