We reviewed the HD500 a couple of years back, our reviewer was pretty impressed, so much so that he bought the unit and has been using it ever since. The new Pod HD500X is something of an update rather than a whole new product. You still get the same connections and basic form factor but it has been tweaked in several ways.
The first and most obvious visual difference it the swapping out of the footswitches for simple, more robust version, "less clanky" are what Murph describes them as. They also have a much brighter and more obvious red LED ring around them - great on stage. Additionally, the LCD screen has been uprated to a brighter version, again aiding on stage viewing.
The biggest difference here is the uprating of the internal processor and there for DSP capability. The Pod HD500 would run into the dreaded DSP Limite Reached message when building too complex a patch - choosing from the 100+ studio grade stomp and effects or the HD amp models With the Pod HD500X - you get around 20% more juice, allowing you to pop in that extra delay or whatever. It translates to around 2-3 extra pedals in practice.
Sound While the actual algorithms don't appear to have been uprated, Murph felt that the sound just felt more impressive somehow, fuller perhaps.
Conclusion Whilst not an earth shattering upgrade, the hardware tweaks all make for a more robust and usable guitar system, the extra DSPallowing for more creative freedom, you can still edit via USB and the Gear