Once you make peace with having to pay roughly one-quarter of the price of the HF100 just for this mic, you'll be very happy with the substantial improvement it provides over the Vixia's internal mic. It's just a shame Canon accessories are always so expensive.
Provides superior audio to the internal mic; Selectable shotgun (mono), 90* and 120* pattern stereo modes; Eliminates wind noise; Lightweight
Typically high Canon list price