Okay, I received my new HF2 yesterday, which from what I understand is the HF5 plus microphone. I decided to sell the ER4P and go with the HF2 for convenience with my smartphone. I bought the headphones primarily for the next phone I'm going to buy, which has a 3.5mm 4-conductor jack just like the iPhone. My current smartphone though, has a 2.5mm 4-conductor jack, and as it turns out, I'm going to have to make my own adapter to get the microphone to work, so I'm waiting on the components to arrive here before I do that, so I can't comment on that.
Now, other than the microphone, my impressions: first of all, aesthetically, the angle of the wire in the HF2 is far superior to the ER4P. Its probably a 100-degree angle as opposed to the 135 degrees or so of the ER4P, so in combination with the small frost tips, the wire basically just follows my earlobe straight down and doesn't look like Frankenstein-style screws coming out of my ears.
So far I've mostly only used them in my living room for testing purposes, so I can't comment on bone conduction, but the microphonics issue is still there, but improved quite dramatically from the ER4P (and mine were the all black ones, which if I remember correctly had a slightly improved cable from the original blue and red ones- I could be mistaken on that though.) The cord is a little thinner than the ER4P, but doesn't feel fragile either.
But anyway, what everyone cares about is the sound quality and how it compares to the ER4P. I've never heard the ER6i, but I would be willing to bet that Synaesthetic's description of them being a "super-ER6i" is pretty accurate. To my ear (which admittedly, I do have a very mild case of tinnitus but otherwise perfect hearing), the HF2 is slightly less detailed in the high-end, which also can make them sound less 'harsh' on the cymbals. The bass is improved slightly, but its still a very clean and tight bass, not the kind that you can feel, so if you don't like the ER4P's bass, you won't like these either.
Basically, I'm very happy with my switch to the HF2. While it's technically not as high-end a model, it doesn't feel like a downgrade but more like a step to the side. While some might be disappointed in the slight decrease in the detail, to me this was a pretty big upgrade, for me the sound quality is improved ever so slightly, and the other aesthetic/practical issues are improved as well.
Edit: a few things I remembered that I should probably point out:
1. I've never used a headphone amp, so I can't really comment on that.
2. I sold the ER4P before I bought the HF2, but my points of comparison are with tracks I've gotten very familiar with over the year and a half I've had them.
3. It's not exactly a apples to apples comparison, because my ER4P had the silver tube filter, and these have the newer green tube filter. I'm not sure if that affected anything at all, but I thought I should mention it.