The Sound Devices 664 is a fantastic mixer/recorder. I primarily use it on video projects, but I have used it to record a couple concerts as well. It sounds great, is very well built, and performs as well as you would expect any Sound Devices product to perform. It is also very easy to operate. If you are familiar with Sound Devices products the controls will immediately become familiar to you.
I don't ever see myself ever needing to upgrade to another recorder. It has everything I need: 24bit, great meters/screen, cf card & sd card slot, digital outputs, 6 analog inputs on full size xlr connectors (with the ability to add 6 more inputs via the CL-6 input expander), time code, word clock, direct outputs, 2 headphone jacks, returns, the list goes on and on....
I highly recommend the 664 to anyone who may be thinking of purchasing one. I was lucky enough to buy mine before the Sound Devices price increase went into effect (saved myself $600).