I found Deshaker as well and it does help a lot. But if your video is really shaky, it will seem to elongate the frames because the difference between the frames in the original video is just so large. The Tachyon XC HD records HD video at 30fps. Normally that's ok, but if the video is REALLY shaky, there just isn't enough frames for deshaker to compensate for the distortion between frames. Maybe a camera that records at 60fps won't have as much trouble with extremely shaky video (like from really rough roads or mountain biking).
If you ride over relatively smooth roads, a helmet mount while using an image stabilization program like Deshaker will work pretty well, but if you ride over very rough roads or are recording very fast paced action, I think a helmet mount will work better because your body will absorb more of the road vibration. You can then use Deshaker to make the video even better.