I have dabbled with various amp sims in the past. I have got a few excellent sounds from some of them but was always disappointed with the response to varying guitar volume (and to a certain extent guitars) as they seemed to be a 'snapshot' of how a certain amp sounds at a certain volume with a certain guitar. The fact that Dweezil (and Hunter!!!) are impressed with the Axe FX would suggest that Fractal have cracked this. If I didn't have 4 valve amps I might be tempted ........ but the thing that attracts and repells me at the same time about the Amp FX is the great flexibilty...I fear I would spend all my time fiddling with it and not playing... then again after a while I suppose once you've got your favourite presets this is not a valid arguement.
Main reason for me to get the Axe was that I loved the sound of the Shiva straight Guitar - Amp - Cab but was never able to reproduce the punch with any effects up front or in the loop. My dream was to get rid of this issue with the Axe, and it came true. Guitar goes into Axe and all else happens in that unit.
On top of that is the flexibility. I couldn't have well kept up with sound expectations with the Metal band using the Shiva head, it just doesn't do that. The Axe does this very well actually, but also more mid heavy sounds too.
Actually one more thing is that amp models in the Axe give me GAS for real amps. I wouldn't have considered a Cornford or an Uebershall, if not the models in the Axe would have been so much fun. Not bought any of those yet, but waiting for the right moment to pull the trigger, just for the fun of it. Yes I am an ampwhore :o)