With a good WiFi connection, the voice quality from the iPhone and iPad apps is nearly as good as a desktop soft phone connection. Not quite as good as a wired, standard phone set connection with the same service (which is excellent), but feature-rich as mobile apps go. It allows me to "be in the office" on the phone without using cell time or revealing my cell phone number, and even to use the iPad in a pinch where theres WiFi, and my iPad has no cellular service.
Now, the addition of SMS messaging means I can text clients without giving out my cell number.
Still looking for a few features in the desktop client that seem obvious to me.
Rick Rutledge about 8x8 Work, v6.3.3