Originally Posted by DR. Rock
I replaced my ER6i's with a pair or the HF5's. I've worn them on all day rides, two week trips, daily use on my bicycle commute, and they fit fine under both the dirt and full face helmets, and sound great. The cord is way more durable than the ER6i's. The filters are different than the ER6i's, so if you've stocked up, you'll take a hit. Also, I wasn't able to find aftermarket filters like you could with the ER6i's.
I am 90% about to return these MC5's for the HF5 particularly if I can get a good deal on an open box pair. After giving the MC5s a chance to burn in, then listening to both sets of buds today, the ER6 is simply in a different league. I believe the MC5s moving coil construction is inferior technology.