With iPhone OS 3, Apple (finally) brought A2DP—stereo Bluetooth—support to the iPhone, which meant we could (finally) listen to music and other audio wirelessly, without the need for awkward dongles. We’ve reviewed a number of stereo Bluetooth headphones, but few have impressed us. Sennheiser’s MM 100, however, stands out.

Fairly unique among Bluetooth stereo headsets, the MM 100 is a lightweight model that uses a behind-the-neck (“neckband”) design with fairly large, on-ear earpieces. At around 2 ounces in weight, with a fit that’s loose enough to be comfortable but tight enough to stay put when exercising, the MM 100 is well-suited for both sitting at your desk and running on the trail. My only complaint about the MM 100’s fit is that, on my head, the spots where the stiff-plastic neckband loops over each ear occasionally became irritated after many hours of at-my-desk listening. But I was able to wear the MM 100 for two to three hours without a problem.

The MM 100’s right-hand earpiece hosts a large, multi-purpose (Play/Pause/Connect/Answer/End) button on the outer face, as well as two sets of smaller buttons—Volume Up and Down on the bottom, and Back and Forward on top—along the outer edge of the earpiece. Near the front edge are an LED indicator and a hidden microphone for using the MM 100 for voice tasks. I like how each control provides distinct, tactile feedback, making it clear when you’ve pressed the button.

Near the bottom of the left-hand earpiece is a small, rubber door protecting a miniature-USB port for charging the headphones. The MM 100 comes with the necessary USB cable, which lets you charge the unit in approximately 2.5 hours using either the included USB AC charger or the USB port on a computer or powered hub. The MM 100’s USB port is the less-common of the two Mini B types, so chances are any other Micro and Mini USB cables you have won’t fit.

The MM 100 paired quickly and easily with each of the several iPhone and iPod touch models I tested it with, as well as with an iPad. (Note that the first-generation iPod touch and iPhone don’t support stereo Bluetooth.) Once paired with a device, turning off the MM 100 temporarily disconnects it from the device, and turning it on automatically restores the pairing. When the MM 100 is paired and on, all device audio—including audible alerts—plays through the headphones.

Although the behind-the-head design is unique among the Bluetooth headphones we’ve tested, what really sets the MM 100 apart from other models is its audio quality. The MM 100 sounds quite a bit like Sennheiser’s popular PX 100, producing solid bass, good midrange, and good overall balance. Treble detail isn’t quite as clear and extended as with the PX 100, but it’s good enough to make for enjoyable listening. The result is a warm sound that’s easy to listen to for long periods, with bass response that’s strong enough to keep you going during your workouts.

(The MM 100’s earpieces are open—they don’t seal on or around your ears to block external sound. So you’re still able to hear what’s going on around you, although this also means that if you tend to listen at high volumes, other people will be able to hear your audio, as well.)

Interestingly, Sennheiser’s specs for the MM 100 include this note about using Bluetooth headphones for watching video: “Due to the nature of Bluetooth technology, users may experience a slight audio latency when watching video on a portable device. The headphones are working fine. This is a characteristic of all Bluetooth headsets. Latency is not an issue for audio-only use.” I’ve read similar comments about audio-video-sync issues from users. However, I didn’t experience any such problems with the MM 100 during my testing—over the course of watching several TV shows and movies on several devices, audio and video synced perfectly.

One area the MM 100 doesn’t excel is as a Bluetooth headset for phone calls when paired with an iPhone. Headset functions all work as expected: You can answer and end calls using the multi-function button—the MM 100 seamlessly switches between headphones and headset modes—and you can even speak Voice Control commands (although I had to initiate Voice Control using the iPhone’s Home button). I could also hear the person on the other end of a call just fine. However, during calls, other people said I sounded worse than when talking directly into the iPhone’s mic—one of my colleagues characterized the MM 100 microphone’s performance as providing little detail and sounding boxy, as if I was “in a tiled bathroom.” In other words, the MM 100 won’t take the place of a dedicated Bluetooth headset, but it’s nice to have the option to take a call when necessary.

As with most Bluetooth devices, Sennheiser claims a range of “up to 33 feet,” and that turned out to be a fair estimate. In testing the MM 100 with an iPhone 4, I was able to walk 32 feet down a hallway—one that included a couple corners—before experiencing any dropouts. The company also claims up to 9 hours of talk time, 7.5 hours of active listening time, or 220 hours of standby time. Although I didn’t officially test the MM 100’s battery life, I regularly used it for several multiple-hour sessions before having to recharge, so these estimates also seem fair.

Unfortunately, when it comes to controlling playback, the MM 100 suffers from limitations of the iPhone’s own software. As I noted when discussing accessory compatibility with the iPhone4, iOS 4 still doesn’t fully support AVRCP (the Audio/Video Remote Control Profile), the Bluetooth feature that lets you control playback using buttons on your Bluetooth headphones or speakers. Instead, the iPhone, iPad, and iPod touch allow only the most-basic AVRCP functions: play and pause/stop. This means that while the MM 100 sports dedicated controls for Back and Forward, those buttons do nothing when paired with any of these devices.

If you really love the MM 100 and want to take advantage of these buttons, you can purchase Sennheiser’s BTD 300i, a Bluetooth dock-connector dongle that lets you use the MM 100 with any iPod but also adds improved AVRCP support to any dock-connector-equipped Apple device. But the BTD 300i adds $130 to MM 100’s $200 price tag. I continue to hope Apple finally adds full AVRCP support in a future iOS update. Until then, I instead recommend installing Bluetooth Helper, an iPhone app that lets you skip tracks using the play/pause button on any Bluetooth headphones or speakers.

One feature you do get with iOS 4 is the capability to adjust Bluetooth volume on your headphones or on your iPhone, iPod touch, or iPad itself. Under iPhone OS 3, your only option was the volume controls on your headphones, which presented a problem if your headphones’ lowest volume was too loud.

The MM 100 can also be paired with any Bluetooth-equipped Mac, with which it can be used as either stereo Bluetooth headphones or as a Bluetooth headset—but not both at the same time. After pairing the MM 100 with your Mac, the device appears twice in the Sound pane of System Preferences: once as Bluetooth Headset, another as Bluetooth Headphones—each mode using a different Bluetooth profile. You must switch between the two modes to, say, listen to music and use the MM 100 for Skype calls. The MM 100 microphone’s performance isn’t any better than when used with an iPhone, but Mac OS X does support AVRCP, so when used as stereo headphones, the MM 100’s Back and Forward buttons work, for example, to skip tracks in iTunes.

The MM 100’s biggest negative is its price. At $200, it’s in the price range of better-sounding full-size headphones, and it’s $80 more than its slightly better sounding non-wireless sibling, the MM 60 iP, which includes a single-button remote and microphone. In other words, you’re paying a good premium for wireless.

Macworld’s buying advice

The MM 100 is the best-sounding set of lightweight, Bluetooth headphones I’ve yet tested. It’s comfortable enough for hours of listening at your desk or around the house, but it’s also a great fit for exercise. The headset capabilities make taking a call convenient, even if they aren’t up to the quality of a dedicated headset. And I like that if Apple ever fully supports A2DP, the MM 100 will be ready, thanks to built-in playback controls. If you’re willing to pay the premium for wireless functionality, the MM 100 is an appealing option.