Hey there - I'm a CA in the Lemoore, California center and we've been using it for a year now.  We were one of the first centers to roll it out, and everyone has agreed from the get-go that it's a major improvement over the old Avaya system.  The platform is a lot easier to use (everyone went "Wow!!  Windows!!!" lol), there is all that help available right on screen, and VCO, HCO, and device to device calls are a lot easier to set up and process.

One thing I have noticed, tho, is that it seems the more they try to upgrade it, the more "bugs" seem to appear...I don't know about any other centers, but our center has been having a LOT of problems with it lately, anything from logging in to screens freezing to text disappearing in the middle of a call.  But I really do think Phoenix is a huge improvement over Avaya!