Guitar Rig 5 is not only my favorite version of Guitar Rig, but it is also the best method of shaping a great guitar sound that I have found in the digital realm.
Obviously a storage room full of vintage and modern pedals/fx processors/amps/speakers/mics is a tone shapers wet dream - but unless you can swing 100k+ in equipment this just isn't a reality. Guitar Rig replaces approx ($) worth of equipment in one smooth running standalone app (or VST if thats your flavor).
After some time with it I've come to love GR5 for not only processing guitar, but for processing everything! The reverbs are excellent, the compressors are as well. The amp sims are very useful to add character to any and all sounds. GR5 is really a great mixing tool - perhaps moreso than it is useful for guitar!
The sounds are NOT true to life tube amp full stack stadium quality, not even close... BUT the unique processing options are extremely useful.
The live useability is something I've explored deeply, and frankly its just not realistic. The inherent delay - the complicated setup - boot times, bugs... etc etc... its just not worth it to run Guitar Rig live. A good multiFX board (the zoom g3 for example) offers PLENTY of processing capability without the hassle of carrying around a laptop/interface device. If I had a crew behind me, I'd make them do the work, but I don't so its not happening!
This does not in any way make Guitar Rig 5 less FUN to use though! The tone shaping options are endless, and amazing results can be had without alot of effort. The sound remains relatively "digital" in nature, but its not inherently bad in any way.
When I use Guitar Rig on my guitar its typically without any amp simulations - processing the clean sound of the guitar and adding reverb/delay etc as needed. I like to use it as a VST plugin and combine it with other plugins (like the classic series here).
Mainly, though, I use it to process any and every sound. The reverbs are very very useful, and the various distortions are useful to add grit and grain to a boring sound. I like especially to parallel the effect using Reaper's plugin wet/dry mix - or sometimes even automate the mix to make things yet more interesting!
Really, this is one of those desert island programs... give me this and give me Izotope Ozone + Reaper and i can mix some sweet sounds.
I suggest trying it out, get the demo here: Native Instruments