Factory turn indicator flashers are designed to flash quickly if a bulb is burnt out. Unfortunately, the low power consumption of an LED looks like a burnt-out bulb to these flashers, which leads to "hyperflash". Hyperflash isn't cool.
You can address the problem by adding resistors, or you can replace the flasher. Replacing the flasher is the right way to do it, but it's a little more expensive. Resistors can get hot, and they do burn a little more energy. Okay, maybe you're not that worried about the latter. But if you want your car to think it had LED turn signals from the factory, a new flasher is the way to do it.
For the NB2 (2001-05), we have a plug-in replacement unit.
The NB flashers are intended for use with LED turn indicators installed on every corner of the car - not only will the flasher not work with incandescent turn indicator bulbs, but there is a chance the incandescent bulbs could damage the flasher.
Please be aware that if a bulb blows out, you won't be able to tell by the speed of the flasher.
Unlike resistors, which are required for each bulb, you only need one flasher per car.