This effect was universal on the first unit I tried; on the second headset, it was barely notable in music and movies, but persisted in games. This could be a result of how the earcups fit on my head, but the effect was definitely more pronounced on the first unit. If you’re playing on PC, you’ll likely be playing with surround sound on, so you can sidestep the issue. But if you were planning to get the G633 because they also work with consoles as a passive stereo headset, you’re stuck with this weird effect. These headphones also don't get very loud when using the 3.5mm cable.
Even with surround on, though, the audio experience isn't perfect. Dolby and DTS Headphone:X both sound good, though I preferred the DTS mode. Dolby's effect is far more subtle than on other Dolby headsets I've used, and DTS's punchiness adds some of the warmth that bass lacks in stereo and Dolby modes. DTS also does a slightly better job at rendering the positions of audio sources. It's just too bad that there's currently so little content that supports Headphone:X.
However, neither option gives a sense of wide, expansive space—whether music, movies, or games, audio felt boxed into a surprisingly narrow area. It's not enough to be aggravating, but definitely discernible if you're used to discrete 7.1 or 5.1 setups, or even other surround-sound headsets.
Chat audio is less of a mixed experience, particularly if you're using it exclusively on PC. The boom mic got lavish praise from conversation partners over Google Hangouts and Steam; one person in a Google Hangouts session said I sounded as good as when on a landline phone. And Logitech's implementation of sidetone, which feeds the sound of your voice into your headset, scales well from subtle to loud with virtually no buzzing. The inline mic on the 3.5mm cable works adequately for phone and console party chats, though chat partners all indicated that my voice was very soft and they had to adjust their volume to hear me. Using the boom mic was worse; I could barely be heard.
Given the lower volume of both incoming and outgoing audio over the 3.5mm cable, one of Logitech’s touted features—the ability to run sound through the 3.5mm and USB cables simultaneously—ultimately doesn’t work as well as advertised. You can certainly take calls with a PC game running in the background, but the USB audio overwhelms the 3.5mm audio. The net result is that neither person on a call can hear each other very easily.
The easy fix for such a situation is to silence USB input by using a switch on the G633, but it's somewhat awkwardly placed and difficult to manipulate while wearing the headset. Using the G633's programmable G-keys and mute button is similar; it's usually easier to pull these off your head to fiddle with. Of the physical controls on the G633, the only one that works well during blind use is the volume dial. It's easy to find and makes pleasant beeping noises as you make adjustments.