Is there some place I can download a demo of Guitar rig
http://www.nativeinstruments.de/index.php?id=gr3demo&L=1
Demo session limited to 30 minutes at a time and you cannot save patches. ☹
any pros & cons I need to know about before purchasing?
I'm the number 1 Guitar Rig slut around these parts, but I don't know if it's going to be
easier than a GT-8 in your application. You need a decent external interface like one of the Native Instruments ones or something like an Edirol - a laptop's built-in interface is, at best, horrible. Depending on your laptop, the GT-8 will probably boot faster than the laptop and has the built in pedal and stomp switches.
On the power side though, GR blows the GT out of the water. For solo playing and practice there are also the built-in "tape decks" so you can do basic backtrack recording or load in drum loops and backtracks to play along with - even speeding up or slowing down (within reason) while keeping the pitch. The metronome is a great practice tool too.
If you're recording in something like Cubase or Sonar, GR is the much better tool to use too - so You would not just be using it for work.