I think they're fun and all, but bloody expensive. I've hrd the sound clips for ebow and can sound really close to a violin sound, not sure if that's what you want in an erhu sound. There's soundclips for the ebow on their website, check it out. The pirahnna one looks like cheaper alternative, basically imitating a real bow. Should do the same thing I think.
I'll rather go for other new gear, since the use of the bow is limited to occasional use(unless that's your thing). Alternative is to use a real violin bow, but I've not quite got around to mastering that, haha anyone who has PM me. Like I said la, its quite ex for what its worth, so try a friend's one if you can before getting one.