a buddy of mine has a studio where he records oogles of metal bands, and uses the PODxt almost exclusively for guitars
the real trick, as some have mentioned, IS TO TURN OFF THE ONBOARD CABINET EMULATION!
then load a convolution reverb, with a good cabinet impulse, and it'll be the closest to the "real" thing that you can get
i also like to try to throw in some sort of analog distortion, or tube preamp in there as well, just to give some sort of flavor on top of the digital processing