The Sennheiser HD 650s are a high-end set of open-backed, over-ear headphones. Headphones with an open-backed design have a much more open, airy quality to their playback as opposed to closed-back designs. The downside to this style is poor isolation and playback that will be audible to those around you. This quality makes the HD 650s ideal for a private listening session at home, where users will be able to reap the benefits of the open-backed design while not suffering from any of its downsides. The HD 650s are also ideal for home use due to their 9 foot-long cord, which can easily stretch across a room to hook up to your audio setup. The cable ends in a 1/4-inch plug, which is ideal for most amplifiers and such. If your device of choice has an 1/8-inch jack, the HD 650s are happy to oblige, courtesy of their 1/8-inch adapter. 

The Sennheiser HD 650s are officially priced around $500, but conscientious shoppers can find them at a dramatically reduced price.

 

Performance  (read the full lab performance results at HeadphoneInfo.com) 
The Sennheiser HD 650s performed well on our audio quality tests. They had an average frequency response score, with an evenly-emphasized bass and mid-tone response. They did fall off slightly at the high-end however. The headphones showcased virtually no distortion, which isn't a surprising result from a set of Sennheisers. The headphones also had very even tracking, meaning the left and right channels remain won't be playing back at different volumes.

As we alluded to earlier, these aren't headphones for use on the go. They won't block out much ambient noise at all, and aren't capable of a particularly high decibel output relative to closed-back headphones. These two factors, coupled with a high rate of leakage, means you should keep the HD 650s in your house. 
 

Comparisons  (read more in-depth comparisons at HeadphoneInfo.com)
Although the HD 650s did well, they didn't blow their competitors out of the water. Even if you find these headphones for under $400, they have some stiff competition. They seem to fall somewhere between the Beyerdynamic DT 990s and the Audio-Technica ATH-ESW9s. While the DT 990s are less expensive than the HD 650s, they managed to perform about as well on our tests. Further, online buyers can customize their DT 990s in terms of color scheme, cord style and impedance. The HD 650s upside is a much more open back. Unfortunately, something like "a larger soundstage," is subjective and falls out of the realm of our testing. The ATH-ESW9s are the upper-class option, offering similar audio quality, but high-end aesthetics. The ATH-ESW9s have wooden backs to their ear cups and lambskin-covered pads.