Got them today (black version from amazon.com, $17) and I'm so impressed I am doing a review on them. These definitely need to be more well known. A forewarning: I don't believe in the drivers burning in and I generally listen to my headphones out of the box immediately. I believe in the brain burning in because during the first 30 minutes, I found them trebly and annoying, but after that I got used to their bright/aggressive signature.
My source is a Rockboxed Sansa Clip+ which I consider ideal for critically assessing IEMs due to the low output impedance and transparent sound quality. My music collection is strictly rock and metal.
Fresh out of the box, I immediately connected it to the Clip+ and began listening to Led Zeppelin IV (FLAC). I was disappointed as I mentioned earlier. The snares and hi-hat were very loud and aggressive. Vocals were pronounced and sibilant, and the guitar was recessed. I could not gauge the bass performance as these recordings are quite bass light. But I could hear everything clearly, despite the guitar being distant.
Then as the album reached the last song (Levee) I experienced aural nirvana. I found myself enjoying the signature. It was bright and detailed to a whole new level. When I say bright, I don't mean like a Grado headphone, but a bit more like the old Sony SA5000. I'm not saying it's as great as the SA5000, but the treble and detail remind me of it. They are like the HA-FXC51 / HA-FXC80 on steroids.
I needed more demanding music so I fired up some Slayer's Reign In Blood (famous for its double bass). The FX40's handled it with ease, which makes me firmly believe that these are V-shaped in signature. Yes the vocals are a bit pronounced (upper midrange) and the treble is pronounced but the guitar did not seem as loud as my reference for tonal accuracy (Etymotic ER4P). It's definitely not for bassheads, because the treble is what dominate the tonal balance, it's for the trebleheads (the very few of us).
If you like relaxing headphones such as the HD650, stay away. If you like analytical headphones, by all means check them out.
Mini comparison with Etymotic ER4P
Clarity is about the same. The treble is far more pronounced on the FX40. Vocals are louder on the FX40. The FX40 is sibilant while the ER4P rarely displays sibilance. The FX40 has more bass but not much more, I'm guessing 3-4 db more. I consider the ER4P the most accurate headphone in terms of tonal balance (yes, I know of the ER4S, but I think the ER4P is flatter). The ER4P is much more refined. The FX40 comes close though.
Mini comparison with JVC HA-FXC51 / HA-FXC80
The bass is roughly the same. But the FX40 is more aggressive sounding on the treble and upper midrange. The FXC51's have a better clarity when displaying electric guitars. I should add that I can't wear the FXC51/80 for long periods of time due to the driver being positioned in-ear.
Build Quality / Comfort
They are built like they are priced unfortunately. You can't have everything.
There is no strain relief and the cord is microphonic (I wear them over ear to reduce this). I am not experiencing any comfort issues like the FXC51/80.
Pros: Amazing value, great clarity similar to $100+ IEMs, satisfying bass response
Cons: Electric guitars sound distant, sibilant (especially on bad recordings), average build quality, can be too bright and aggressive at times
I will update my review as I listen more with these. Thanks for reading.