I just received my pair of H-300s the other day and have been A/B'ing them extensively against my RE-272, HE-500, and ATH-M50. I will admit that going in, I was hoping for (without any reason to do so), something akin to the RE-272 with a high quality / stronger bass presence. Here are my brief impressions thus far.
the HIGHS (emphasis added):
Full disclosure - I used my RE-272 and HE-500 on a daily basis before this arrived. Call me spoiled by the HiFiMAN house sound, but the H-300 is downright harsh and sibilant in comparison (even the exaggerated highs of the ATH-M50 are tame by comparison). I think the RE-272 and HE-500 are every bit as detailed, defined, and extended at the top, but provide a smoothed out pleasant sparkle. Using the included tips (both foam and silicon), the 8-9k spike (ouch!) is a borderline deal-breaker for someone like me accustomed to the HiFiMAN sound. Luckily, I've been able to tame this down considerably using some spare Comply T-400 tips I had on hand. Out of the stock tips, the black silicon tips with the colored cores seem to do the best job at taming the spike.
That said, this pair is significantly brighter / hotter than all of the above sets.
Mids:
The RE-272 and HE-500 have much more body, thickness, and fluidity to the mids. This might not come as a surprise to people familiar with HiFiMAN. What the H-300 lacks in body, it seems to make up in clarity. Compared to the RE-272, the mids sound much clearer, and exhibits better separation in the mids. The clarity almost reaches the levels of the HE-500, but is much thinner in comparison. I say it "seems" to make up for the thinness with clarity because I'm not currently certain whether this apparent clarity is due to the stronger top-end presence, or if the H-300 is actually besting the venerable RE-272 in terms of clarity and airiness.
Lows:
Wow, you guys weren't making this stuff up. It lives up to the hype! The lows are spectacularly rendered. Could not be happier about this. Tight, quick, resolving, extended, they meet all the claims in this thread. I think the lows are almost, ever so close, to the quality of the lows in the orthodynamic HE-500. During all my sine-wave sweep testing, I could not stop marvelling at how close the H-300 was to the HE-500. I do believe the HE-500 is slightly more extended, and gives more of a sense of definition, but only slightly. The presentation of the low end is not overwhelming either, I'd say just right.
So obviously, this paints a picture of the H-300 as being quite U-shaped, as others have reported.
Relative to each other, the RE-272 seems about as ruler flat as it gets. The HE-500 again seems flatter, and much more mid-centric. The ATH-M50 (I think it's a little outclassed here) sounds like it has a similar sound signature (with tamer highs), but is bested by the H-300 in every area in terms of sound quality.
I really wish they brought the mids forward a bit and given them a bit more body and tamed the 8-9k spike a bit. However, right now, with the Comply tips, the highs are just about right, and only sound harsh on very few songs in my collection now.
That said, I prefer the H-300 over the other pairs in this comparison for pop-ish, electronic, more modern alt/indie/pop rock, certain acoustic music. Vocal, jazz, instrumental, classic rock, live music, classical, are tipped in favor of the HiFiMAN pairs in my collection. For this reason, I see the H-300 taking the place of the ATH-M50 in my collection.
Just my 2 cents. YMMV.
Source info for comparison:
Home: ODAC standalone into O2 amp
Portable: Samsung Galaxy S4.
Misc:
I saw a post earlier asking about sound differences between cables, and I don't notice any difference when running from my Galaxy S4 (black mic cable won't work on O2).
I also noticed some mild driver flex when using the supplied red foam tips.
(tho at this stage, no one will read anything radically new, since I procrastinated about a week too long on it 
)
