I just bought a pair of these tonight because my ER4Ps are starting to go and since they're over 2 years old and I would have to pay for a repair, I decided to use this opportunity to get the hf3s.
Previous, I had purchased the mc3s because I wanted something with iPhone controls, but I never really loved them. They sounded ok, but I think I was just spoiled by the ER4s, which sound amazing even without any kind of external amp. Also, the mc3s were much bulkier and I never really found them all that comfortable.
The hf3s sound fantastic. I've always preferred accuracy over bass, and that may put me in the minority, but to my ears these are perfect.