My personal favourite headphones. I measure all my future headphones to this pair.
 
Tight bottom end, punchy, great clarity in the highs.

I was hesitant buying these closed cans. I've read everything from floaty bass to decimated vocals. However I've learned that you can only believe what you experience yourself.
 
I was also bit hesitant with these cans because their frequency response curve was a bit amped over my baseline AKG K701 / Sennheiser HD598. However, I wanted to find a closed can that was close to the HD598 with a bit accentuated bottom end. What you'll find is these cans have an odd signature in the curve.

 
What I found is these cans produce a large clean sound. The bottom is punchy, but not muddled however you may find at louder volumes you may want to moderate how much you use these cans as they can be quite bottom end responsive. Be aware there is a PRO and a CONSUMER variant with these headphones. Black are the PROS and grey is the CONSUMER. The Pros have a bass port to allow the diaphragm breathe.
 
However! This bottom end doesn't come at the expense of losing the mid and highs. In fact, I found that the high end was nicely smoothed so you shouldn't find much shrill with these.
 
What I really enjoyed with these headphones is they were very adaptable to a wide range of genres.
 
The headphones are light, well constructed and very comfortable. BUT... not necessarily those with endowed ears. Beyerdynamic chooses to use a circular design versus a oval like Sennhesier so those with large ears like me may find a break is needed to ease the long periods of listening since you can get a bit of ear pinch. I found though the band clamping is not excessive.
 
Last thing on the headband, that as it springs open it clamps the earphones so to adjust it's best to allow the headphones to close. I think this is a design that carries through the DT series of headphones (880, 990).
 
Driving these phones I used  cMoyBB and I was surprised that my iPhone 3GS worked fine as well. However I would recommend you amp these cans to properly drive them since you want to take advantage of the higher resistance of the 250 Ohms (there are 32 and 80 Ohm versions available).
 
Bottomline:
 
If you want a closed can, you will be hard pressed to find a better sounding set of headphones that are cheaper.

I decided to ditch my Shure E105 because they were just all midrange and even though a tight bass the bass had no presence.
 
Enter the Monster HP In-Ear Speaker. Yeah Monster is big on calling them speaker not headphone.
 
What struck me immediately was the clarity and punch. These definitely have punch and depending on the type of music you listen to it can be very easy to get enveloped in such richness. Take for example the track Monte Gargano by the Truckfighters, wow. Super kick drum and just full sound and while they are stoner/fuzz rock you could still clearly hear the highs on the high hats.
 
I have 2 tests for testing the clarity, presence for punch, clean sound, full range and crips highs:
 RUSH - Tom Sawyer
 Opeth - Windowpanes
 
Tom Sawyer tests the clear punch bass of the kick drum and bass, and the mid-range roundness for the drums and crisp highs.
 
Windowpanes has similiar attributes but the drum work and specifically cymbals and high hats is a great test for that tight cymbal sound without fatigue or loss.
 
The biggest surprise was when I put on some Oscar Peterson Trio. Yep these are great jazz IEM.
 
These Monsters passed. Great punch and the clarity on the highs is great.
 
The biggest caveat is fit. It is crucial to get a good fit in the canal to experience the best sound. I found the tips a bit awkward to find the right fit but you will find ones to settle on but you'll need to experiment on in-ear fitting. 
 
The wiring is thin, so I don't expect to use these in physical activity, so these will be office use. I liked the Sennheiser I had, very beefy wiring.
 
Lastly, I hope they burn in a bit, because the bass can be a bit much for a very long listen.
 
-- Update 28 May 2011 --
So after using these for a while I found that using the aftermarket tips made a big difference in getting them to fit more easily. I have to say though that these do not respond well to compressed music especially if you like using services like RDIO or MOG. The frequency crunch is too much and leaves these phones sounding flat. After some use as well I find they lack a bit of high frequency clarity but that can be acquainted to different music genres too (i.e. some rock lacks the high end clarity, and will emphasize the mid/bass range a bit to much).

This is my first tube amp for headphone listening, and is also my first piece of kit for my listening upgrade. That being said, the Valhalla sounds great with my lackluster headphone output  from my iPhone and using the less talked about V-Moda Crossfade LP headphones.
 
As you can see, the Valhalla is going to compliment almost you put to it. I noticed an immediate stage and sound improvement and this while using lossless (FLAC) through the amped headphone output. I truly look forward to using the amp with at a minimum line-out / digital output source.
 
I've tried the Valhalla through fuzz-rock, jazz, heavy metal, vocal songwriter genres and it has performed very well. Unfortunately, the V-Moda are tuned a bit to the low end so I can't speak of the highend veiling that @Skylab mentioned, but even with these headphones the highs were still clear and distinguished. I am looking forward to testing with my newly ordered Sennheiser HD598. The amp definitely opened up the bottom end and gave a nice large sound.
 
As a test, I tried some lowly Sennheiser HD201s and you definitely noticed the extra volume required by the amp going from a 32 Ohm to 24 Ohm and from what I've read from Schitt is the Valhalla's sweet spot is a 300 Ohm set like say AKG 701/702
 
 
-- Edit --
 
I've since been using the amp with my AKG 271MKII and Sennheiser HD598 and I'm very pleased with this amp.