My experience is that while hardware units
(like the POD) are great for playing live and at rehearsals, the software amp-sims have MUCH better sound and MUCH more sound-shaping possibilities.
I use
'ReValver' from Peavey myself, and the sound is incredible. In a mix, there is no one that's going to hear the difference between your tone and a real amp, it's just THAT good!!
However, 'Guitar Rig' is not one of my favorites. It's much too 'clean' to sound anything like a real amp
(although you can get some great sounds out of it). A much better option is
'Amplitube 3' from IK Multimedia or
'ReValver III.V' (the best!!) from Peavey.
These programs really DO sound like real amps
(and you get tons of amps, speakers and effects to work with).
Of course, for playing live, I wouldn't recomend a computer-based set-up. But when it comes to recording and playing at home, the software amps have taken the lead for sure!! You DO need a strong computer with a very low latency soundcard though...
There is even a FREE version of 'Amplitube'
http://www.ikmultimedia.com/products/amplitubecs/ , which you can update later with individual purchases from the 'Custom shop'...
Peavey's 'ReValver' is quite expensive
(but woth every penny), but you can install the entire program
(without save-function) to hear what the fuss is all about.
http://www.peavey.com/products/revalver/ Give them a try before you decide!