I have GR2. For the most part it is the same as GR3 with a few less enhancements. I really enjoy it. I too am not an audiophile- nor am I an experienced gigging musician. The extent of my stage performance is limited to a few open mics with several drunken college buddies many years ago. So, everything I play is for my own head- in my own space.
I mostly use it with monitors on my desk or headphones into my interface (M-Audio Black Box)... I do have Rig Kontroller 1 that I no longer use because I no longer have (a functional) input box with dual inputs. RK 2 and 3 were later redesigned with USB 2.0 interfaces. RK 1 used two 1/4 in. balanced outs to interface with the software... one was for the guitar signal and the other sent control tones to the software to operate the foot switches and expression pedal. The switches and pedal can be assigned to any effect in the package. RK 1-3 all have built-in pre-amps.
I do agree that GR achieves some very fine tones. The overdrives and distortions are definitely the best sounding models and probably why most of the amp modelling software that I have read about or demo'd seems to appeal to mostly metal players. I have attempted to reach out to players in some of the amp modelling forums to exchange ideas about build effects chains for Jerry or Bobby's throughout their various eras. I don't get any responses though : )
It is fun though... you can experiment with building chains and achieving tones but without having to spend $$$ for each stomp or rackmounted effect. Sure, the tone of the software modelled effect is not going to sound exactly like the real thing. But it depends on what you are using it for.
Anyway, thats my two cents on it. Basically I think its a great creative tool with very passable tones. Its versatile but does not replace the 'real thing' when it comes to effects.
This guy won a Guitar Rig 3 contest sponsored by Native-Instruments with this video. Check it out: