Call quality largely depends on your paired phone, the network that you're connected to, and the quality of the speakers that you connect to the Sony MEX-GS600BT, but callers that I spoke with during my testing said I could be heard clearly.
The Bluetooth connection also enables wireless audio streaming via Bluetooth A2DP for phones and devices that support the profile. Rudimentary AVRCP transport controls, such as skipping forward and back, are supported via this connection.
The perplexing App Remote mode
The Sony MEX-GS600BT has a party trick that is sort of awesome, but also sort of weird. I'm still on the fence about its usefulness, but allow me to explain how the App Remote works.
The App Remote is an app that is downloaded and installed on an Android or iOS device that gives the user two-way control between the in-dash receiver and the apps on the smartphone. I did my testing with a Bluetooth-paired Samsung Galaxy Nexus running the latest version of Android Jelly Bean, but the functions described below should work identically with an iOS device via USB.
Once launched and connected to the receiver, the app presents a bank of shortcuts for the receiver's available audio sources that you can tap to quickly jump between. Additionally, you can add shortcuts to your favorite audio and navigation apps. There are no limitations on what apps can be added, so there's nothing but your own good judgement stopping you from adding Facebook or Temple Run 2 to the list of shortcuts. (Please, don't.)
Once you've got your shortcuts set up, tapping the Source button on the Sony MEX-GS600BT's faceplate will bring up the App Remote app and allow you to quickly choose any of these shortcuts by either twisting the control knob and tapping to launch the app or on-device source or by swiping and tapping an icon on your phone's touch screen. The app names are even reflected on the Sony MEX-GS600BT's LCD screen, which is a very nice touch.
On paper, the App Remote mode sounds cool, but it has one major flaw: it lets you jump between apps quickly, but you'll still need access to your phone's screen to interact with those apps. Unless your audio-streaming app automatically starts playing when launched, you'll still need to tap "play" on, for example, Spotify. It reduces driver distraction, but doesn't eliminate it.
I found it useful to mount my smartphone near the MEX-GS600BT's home in the dashboard, add shortcuts to my favorite podcasting apps, audio-streaming apps, and navigation apps, and use the source-selection button to quickly jump between viewing a map when navigating and picking a playlist while simultaneously listening to audio.
Other audio sources
The Sony MEX-GS600BT's full array of audio sources can be accessed via the App Remote or -- obviously -- through the on-unit source selection menu.
The receiver features AM/FM radio tuning, CD playback, and USB audio playback of MP3, AAC, and WMA audio with the ability to browse folders via the unit's menu. When connected via USB, an iPhone or iPod device's media hierarchy can also be browsed and played back via the Sony MEX-GS600BT's interface. Of course, the 3.5mm input on the faceplate of the receiver allows simple analog audio input from any auxiliary audio source.
Like many advanced car audio receivers, this Sony unit also features PandoraLink control of the Pandora Internet Radio app. Once it's connected to a phone running the app via the appropriate connection (USB for iOS devices or Bluetooth for Android phones), you can browse your preset radio stations, skip tracks, and give songs a thumbs-up or thumbs-down. The LCD does a fairly good job of displaying as much of the metadata for the currently playing song as fits within its 16-character limit.
However, the unit's dependence on the bank of identically small buttons with even tinier labels beneath the LCD means the thumbs-up and -down functions aren't as easy to access without looking as those actions are on competing models from Alpine, for example. You'll be squinting at the screen the first few times you try to access the function and may find yourself accidentally rating a few songs incorrectly. It's not so bad that it's frustrating (I simply abstained from rating songs while driving), but it bears mentioning.
The Sony MEX-GS600BT is a good way to add Bluetooth hands-free calling, a modern array of digital-audio connections, and a rudimentary level of smartphone app integration to any car with space for a single DIN receiver -- which, generally speaking, is all of them.
App Remote mode is cool, but it is basically simpler app switcher. By itself, the function doesn't really eliminate driver distraction from apps versus a naked Bluetooth A2DP audio connection. Combined with a dashboard mount for your smartphone, the App Remote does reduce the amount of time spent looking at the screen and tapping tiny icons when, for example, firing up your favorite navigation app.
At about $200, Sony MEX-GS600BT is also a relatively inexpensive receiver and is a good value when you consider all of the value added. The low-profile appearance will likely draw less attention from most would-be thieves than a $1,000-plus double-DIN receiver would, and the detachable faceplate will deter the rest.
I should also mention that the Sony MEX-GS600BT includes a small IR remote control that replicates many of the controls from the unit's faceplate. Who actually uses these things!? Let me know in the comments below.