A Small Comparison: HF3 vs RE0:
Treble:
HF3 - Beautiful treble that is just a little too forward in my opinion. The HF3 sound amazing with percussive sounding acoustic instruments - acoustic guitar and especially piano.
RE0 - Nice treble that you don't even notice after a while - extremely balanced with the rest of the sound.
Winner: HF3
Mids:
HF3 - I'm listening to Janis Joplin's Pearl, and the organ, guitar and piano have such a great, full attack on the louder parts - extremely crisp and fast.
RE0 - Great mids that don't offer anything that stands out to me especially. The present everything in a balanced manner.
Winner: HF3
Bass:
HF3 - The bass really is too low in the mix. I expect I'll get used to it from what I've read, but, even coming from the RE0, not exactly bass monsters, there isn't isn't much bass. (yes, I have a good fit)
RE0 - I'll say it again: balanced. Don't expect to be blown away by the depth or quality of the bass, but it's there, and there is enough of it.
Winner: RE0
Soundstage:
Both the HF3 and the RE0 manage to separate instruments nicely, but the RE0 simply has more separation. This makes for a more analytical sound. The HF3 is a little more punchy, but there is less width to the soundstage.
Winner: Tie
Fit/other:
HF3 - Good fit, although the only tip that works for me is the large triple flange. The packaging is nice, with a carrying case, instructions on how to get a good fit, and 3 pairs of extra tips. The play pause button is very useful - if anyone is wondering whether to go for the HF5 or HF3, and owns an iPod, I would heartily recommend the HF3. Oh, and the isolation is amazing, but you already knew that. I can play my iPod classic at 1/4 volume while on the highway (not driving) and hear every detail. (with the Red Sox game blasting on the radio)
RE0 - The fit is great with all the tips for me, but sound quality varies greatly with all of them. I was able to get the best isolation with the comply foam tips (not included). The tips included do seem like they were simply what was lying around the factory; there's no sense of them being tailored to the earphone like the HF3. Packaging is okay, but there is no case included
Winner: Tie
Important: The RE0 is not a good earphone to be rough with, as it is not very durable! Do not use it as a main earphone if you want to sleep with them on, carry them around in your pocket, etc. The HF3's durability remains to be seen, but Etymotic has a 2 year warranty, as opposed to Head Direct's 1 year.
Overall Winner: so far I like the the HF3 better, but your mileage my vary; thee are both really nice IEM. If you like a more balanced sound, go with the RE0. If you like slightly more attack, and don't care about slightly less bass, HF3/5 all the way. If you plan to use these on public transportation/an airplane, get the HF3, if only because the isolation is just so good.