Unlike the DSP-500, this isn't USB and so sports two jacks on the end of the cord to hook into your sound system, whether integrated on your motherboard or a discrete sound card. The dongle on the cable has controls for volume and microphone mute, as well as a clip to hook onto your shirt. As a matter of personal preference we're not great fans of dongles, due to the fact they often act like a pendulum and swing distractingly when you're trying to game. The volume control is also nothing more than a variable resistor that can distort the signal, leading to a reduction of audio quality. However, if this doesn't bother you, then you might find the handy volume control useful - especially for muting and unmuting the mic in-game.
In terms of audio quality, the headphones are pretty darned good, far better than the Plantronics. They're not what you'd call high fidelity, but they are perfect for gaming, with strong, boomy bass that is great for recreating the gun battles on screen. However, the high end is definitely a little lacking and if you intend to listen to any music with these, you won't be able to hear the subtle nuances in music by artists like Wishbone Ash or Lemon Jelly, but you're sure to get a rocking experience out of Green Day and Funeral for a Friend. These are not headphones for the dedicated music listener, but they are pretty decent nonetheless.
The quality of the mic, however, is not quite as awesome. Our tests in Skype and XFire suggest that it's clear and great for gaming quality comms. However, the input isn't as clean as on the Plantronics which, to be fair, sports a mic far better than its price suggests it should. If you are looking for a headset for podcasting, for example, we'd definitely say to go with the Plantronics. If you go for the Sennheisers, we also found that the sound was better with the foam padding on the mic removed.
All in all, the Sennheiser headset is good value for money, comfortable for extended gaming sessions and will give you great quality for the price. If you want the absolute best in mic and audio quality, however, look elsewhere.