In terms of technical specs, the ATH-M30 headphones feature 40mm large-aperture drivers with neodymium magnet systems. As with the ATH-M50 headphones, I ran an eclectic mix of music through them and they sounded good with everything I threw at them (they're designed to play well with just about any kind of music -- or movies and games, for that matter). While they don't offer the level of detail, more-refined bass, and overall more dynamic quality of the M50 headphones, they deliver well-balanced, relatively open sound for closed-back headphones (by "open" I mean the sound doesn't feel canned or trapped inside your head).
That bass may not be quite big enough for folks who are looking for a booming low end with big thump, but I gravitate toward headphones that are a bit more laid-back (warmer) and have flatter, more natural sound. These guys certainly fit that description.
As with the step-up M50 headphones, I had both editor Justin Yu and contributor Steve Guttenberg listen to these. They both prefer the M50, of course, but also thought the M30 sounded quite good, especially for the price.
There are several other "studio monitor" headphone models in this price range, including Audio-Technica's own ATHM40FS, which requires a 3.5mm adapter for mobile use and weighs slightly more. That model also has a extra-long cord. The Sony MDR-7506 and MDR-V6 are other popular and highly rated sub-$100 options, but they cost more.
Many of these full-size, closed-back headphones weren't designed with an emphasis on mobile use. But with larger headphones being in fashion these days (thank Beats for that), more people are taking a look at affordable models like the ATH-M30 and ATH-M35 to use for listening in the office (they do seal out sound, so they're good for cubicle use) and at home, as well as on the go.
Alas, I wish Audio-Technica made these with a shorter cord (and perhaps provided an extension cord for "studio" use), but that's the only serious strike against them. Otherwise, they deliver a solid mix of comfort, decent build quality, and impressive sound for an affordable price.