I use my 500 as my pedal board using the 4CM (4-cable method). It's awesome: basically, you plug your guitar into the Pod, use whatever stomp boxes you want in front of the amp, then create an "FX Loop" box, plug the Pod's FX Send into the front of your amp, plug your amp's FX Send into the Pod's FX Return, then plug your Pod's output into the amp's FX return.
Works great with my Mesa Mark V; there is a noticeable amount of "tone suck" but it's not a deal-breaker. It's to be expected with any A/D/A conversion...
I've got no idea about using the pod to change your amp's channels... it'd be nice the Mark V had that capability; then I'd have one of the foot switches change from my amp's OD channel to the clean channel and kick on a compressor and phaser (or something). That'd be great.