Pros: Custom-tailored frequency response contasts notes better, meaning, they're easier to play in. Which is why they're often used for playing synths. Also very useful for sniping dirt in samples and fine-tuning mixes.
Cons: Awful stock cable.
AKG K-240 Studio are at least the 4th (really 5th or 6th?) iteration of K-240 design, which started with the K-240 Sextett in the 1970s, themselves a circumaural version of supra K-140. The basic idea behind the "Sextett" design is that the main diaphragm drives six passive radiators, which enhance ambience and spatial imaging. Now, while this is a very good idea for headphones in an era when most headphone amps were powerful enough to drive 1200-ohm and even 2400-ohm headphones, unfortunately modern amps have not been able to keep up and thus the K-240 and K-271 have a reputation of being power hogs. Really modern device (cell phones', pocket players', netbooks'...) headphone outputs are too weak and lame though. They're still efficient enough to be driven fine (thoughly slightly weak) by most Apple players.
Classic K-240 (Sextett, Monitor, DF) are all 600-ohm. The K-240 Studio are the modern 55-ohm version, lower impedance, higher efficiency. While they work with anything, really they have lost some of the smoothness of imaging and stability that 600-ohm offers. Still, they can be driven by pretty much anything nowadays. The only issue is that weaker pocket players and notebook PCs won't be able to drive them to fullness. They work just fine with any professional gear though, which is what they're meant for.
Make no mistake, the K-240 Studio are designed for use by musicians and sound engineers. Which is why they have a frequency response which is not straight by definition: midrange is raised to improve note contrast and show details in recorded takes, mixes and samples. It is not carelessly lifted though, and the K-240 still manage to have a very musical and rather natural sound signature, unlike their sibling the K-271, which has a weird treble/midrange transition and an overall melancholic character. The only advantage the K-271 Studio have over the K-240 is being fully closed; if you're trying to choose between the K-240 and K-271 for listening to music, go for the K-240, they are warmer and more natural.
Sometimes the K-240 are said to have a "sloppy bass", but this might be more an issue with amplification (the K-240 by design are not very efficient, having passive radiators that are driven by the main diaphragm too). The K-240 Studio are not designed as a boombox, but with the right amp they can deliver a lot of low-frequency power and massage ears with just about any kind of music, as long as it has low frequencies.
High frequencies can feel a tad too rolled off, but really this is a design decision as it seems AKG decided to roll some K-240DF properties into the Studio design too, bringing the overall frequency response closer to the DF model, while still keeping some HF response. As a result the K-240 Studio can sound darkish at times, but they're still sparkly and 3D.
Where the K-240 Studio really shine is space definition. As an example, stock-cabled Denon AH-D1000 will play a rock piece with the guitar in the centre, bass right next to it, and drums in the background, the singer floating in mid-air. In K-240 Studio, this same piece plays with the guitar in the centre-sides, bass separated better and tugged to the back, and drums panned all across the soundstage. The soundstage is quite large, and it's defined rather well to any side, even vertically. The K-240 like to separate instruments and pan them properly. This might also be an important quality for any classical music listeners.
The one big flaw of the K-240 Studio is the stock cable. Some say it's made of leftover old scrap copper, and to tell the truth, it sure sounds like that. The stock cable just won't make the K-240 show all they're capable of. Too dirty, too many stuck harmonics, too dark, too slow, bass and high frequencies slightly falling apart in the time domain, and of course, the delicate high frequencies get lost too, simply because the stock cable is not shielded. To make K-240 Studio show what there really is in a record, you need a good headphone amp (or output; a stationary stereo system amp will do, as will a mixer, a good pro sound interface, etc.) and a shielded high-quality cable. Shielded high-purity copper is one option, but they really become accurate with a shielded silver-plated copper cable.
Dynamics are slower than Denon/Foster headphones, but still quite good. K-240 aren't as fast, but they still paint with a good accuracy. They could be a bit sharper in dynamics, but with a silver or silver-plated copper speed/response is improved.
K-240 can be used for mixing, though really they're mostly for "zooming in" on parts of the soundscape and discerning details that are not as clear on speakers. They're very helpful when cleaning up samples and microphone takes. Like a "magnifying glass" for sound. They also work fine for monitoring. Don't let the "semi-open" bit fool you - even a singer in front of an LDC won't let music bleed into the microphone. A producer friend uses the K-240 Studio for singers in front of his C-414. They might bleed something when cranked up way too high (which is difficult), but in practice they're fine for any kind of studio work, though frankly closed headphones are still better for monitoring microphones, as they block more external noise. On the other hand, you can have radio talkers hear each other "live" and their own voice without taking off the headphones, which might be a downside or a plus.
The K-240 are built tough, from a dense plastic that's guaranteed to hold (mine once got bumped by a moving car). The "Studio" name is really appropriate, they're studio gear meant to last.
Overall, highly recommended. This is a must-have for anyone who plays synthesisers, edits sound, composes music, and mixes. Just recable :-)