These headphones are a great bargain for anyone wanting a very good monitoring headphone that delivers great SQ across the frequency spectrum.
 
This is not the most fun sounding headphone since it is designed for monitoring, so if you like your music with a decent bit of coloration, then you should look elsewhere.
 
That said, it is not completely neutral.  It does have a somewhat warm midrange that makes it fantastic for listening to vocals.  Overall, for the price you pay, these cans offer a decent amount of detail and an overall balanced sound that should please audiophiles who are on a budget.  Bass is accurate and tight, but a bit on the light side.  As others have said, they can be a bit sibilant on some recordings.
 
Impedance is 44 Ohms, but it is fairly easy to drive from a laptop or mobile devices/MP3 players.  
 
The only niggles with this great pair of cans is that they are a bit heavy.  That's not to say they're uncomfortable (they are fairly comfortable), but they have a good deal of mass.  Overall they have a very sturdy build, but I didn't like the exposed wires above the earcups.  Thought that was a bit of an oversight on Shure's part and could lead to damage if you're not careful.  But if you treat your audio and tech toys with reverence and care (like I do), you should be fine.
 
After 2 years of using them, I would highly recommend these.  They have a very good reputation both here and elsewhere online.  For the money, you cannot beat the clean, detailed sound these offer.

When I went looking for a new IEM last year, I really did a lot of research and wanted something with great SQ and detail but also fun sounding as I listen to a lot of rock and planned to use this pair at the gym.
 
These are an absolutely fantastic value.  They can be had for around 80-90 bucks online, and for that price they are absolutely wonderful IEMs. Check out the excellent review here on Head-Fi in the 309 IEM comparison thread for a more detailed review of these.
 
These are not analytical phones by any means, but they provide very good detail and imaging, excellent timbre, and are flat out fun to listen to.  Not a basshead IEM, but the bass does have good impact and authority.  The presentation is very intimate and somewhat in your face, but I liked that aspect of them for what I was using them for.  They have good energy and kept me pumped during my workouts.  
 
The somewhat U-shaped frequency range of these makes them excellent for rock, which is mostly what I listen to, but they work well with other genres as well.  
 
Isolation is okay but not great, but that's to be expected given their design and relatively shallow insertion.  They are pretty comfortable, but not the most comfortable IEM I've used.  Build quality is very sturdy with a semi-translucent shell that lets you see the two carbon nanotube drivers inside.  They are tough little drivers and do take a bit of a burn in period to loosen them up.  Mine didn't start to sound their best until after a couple months of use.
 
Sadly someone stole them last month, and I ended up trying a HiFiMan Re-400 as a replacement, but I would highly recommend them, especially to rock fans, and I definitely may pick up a pair again down the road.