Haven't used it at all, but I have heard some clips posted over on the Line6 forums in "compare and contrast" type threads up against the PODxt. Most of those sounds have been pretty good -- not going to fool everyone all of the time, but not bad by any means.
It's a Line6 forum, so 70% of the people scream "My PodXT sounds just like my friends $4000 6-tube, multi-gain staged head, you loser", 20% say "Update your firmware to 2.0", 5% yell "Troll" and the other 5% who are objective don't hear much of a difference.
I own a PODxt. The sound difference between it and a mic'ed amp is chalk and cheese so I will always record so I can re-amp later. As such, I didn't hear enough of a difference to justify shelling out more cash for modelling technology.
Still got yer fvcking 19 inch 800lbs Curtis Mathis TV? Yea...can't beat tha fvcking quality of tube. I'd take that over any hi-def flat panel digital tele any ole day.
I heard good reviews from my friends for NI, but for that price I rather get POD XT, with which you can travel and play live And it does not tax your CPU