As for other specs worth highlighting, this is a Bluetooth 3.0 device that has a built-in rechargeable battery rated to deliver up to 7 hours' use.
Like a lot of companies, Denon is touting a set of free companion apps, including the Denon Sport app, that allow you to track your workouts. I should also point out that the headphones ship with a very nice carrying case that's equipped with a detachable carabiner. To protect your investment, you'll want to keep the headphones in that case when you're not using them.
The sound quality of wireless Bluetooth headphones has improved over the years, but Bluetooth compression schemes often make your music sound less dynamic and clear. I can't say I was blown away by the sound quality of the Exercise Freaks, but I thought they sounded good for Bluetooth headphones, offering decent clarity and bass response so long as I got a tight seal. From a performance standpoint they were neck and neck with or even slightly better than the JayBird BlueBuds X, which cost around the same price, and overall I preferred their fit slightly more.
Bluetooth has a range of about 33 feet, so it's possible to stream audio from a Bluetooth-enabled device to the headphones at up to that distance (and sometimes even a little farther). But Bluetooth headphones are prone to the occasional dropout, and from time to time my music would very briefly cut out.
Using the Freaks as a phone headset, call quality was decent, though not great. I could hear fine, but in polling a few callers, the verdict was that my voice sounded slightly muffled.
When judging sports headphones, one of the biggest things I look for is a comfortable, secure fit. Sound quality comes second, followed closely by durability, which is much harder to judge because I'm posting this review after using the headphones for only a couple of weeks. They held up fine after my handful of workouts on a treadmill, in which I varied my pace between 6- and 10-minute miles, working up a good sweat. However, I can't tell you how well they'll hold up after months of use.
As I said, the Exercise Freaks sound good for Bluetooth headphones. There are other competing models that cost less -- the aforementioned Plantronics BackBeat 903+, as well as certain JayBird models and the Arriva Leo. For people on a budget, those may be better choices.
The risk you run with these is that they're not the type of headphones that will offer a universally excellent fit and experience for everyone. And when you're spending $150 for a pair of headphones, you tend to have fairly high expectations. For some people, the Exercise Freaks will live up those expectations. But for others, they won't. So your safest bet is to buy them from a retailer or e-tailer that offers a good return policy, for these are headphones that you should ideally try out before totally committing to them.
Editor's note: This review was updated on 2/11/2014 to reflect new entries in the wireless sports headphones market. The design and features ratings dropped a point. The value rating remained the same despite a drop in price (you can now get these online for closer to $100).