I am a long time Line6 user starting with the origianl POD, and currently using a POD XT/Live. I have been anxiously waiting for Line6 to update the POD HD line. Here's my view on the POS HD500X.
I'm a flexible musician in that I play a number of instruments including electric guitar (Strat, Les Paul and Variax 300 with MIDI capability), acoustic guitar (Variax 700 Acoustic), mandolin, and bass. I like Line6 products because they provide the flexibility I need to be a flexible musician. The POD X3/Live provides mic and preamp models that support acoustic instruments, as well as a multitude of guitar and bass models. I use the X3/Live with a Roland AC90 for flexible sounds at low volumes, and a modified '65 Fender Showman for larger venues. The Showman has HI-Fi tone controls in the Normal channel, and full range coaxial speakers in a Tremolux cabinet to support the wide range of instruments.
My problem with the HD500 is that it doesn't support this flexibility. There's no mic preamps, acoustic guitar amps, or full range preamps like in the X3. And there's only one somewhat limited bass amp. The HD500 is too focused on just electric guitar.
I had hoped the next generation POD HD would address these limitations at least with a few more models. I'd happily drop the $500 if it replicated a bit more of the capability of an X3 with wider dynamic range and better models. But the current HD500 line doesn't meet my needs, so I'll have to stick with the X3 a while longer.