Well, where to start. I've had this product for a couple of months now, and have barely scratched the surface of what it is capable of, but what I have achieved sounds amazing.
The collection of iconic/classic guitars have been sampled very well, in every articulation possible. Add to this the scripting that lets you play from fully damped to full sustain smoothly (using either velocity or mod wheel), the collection of varied and realistic release noises, etc. and you have a very capable product.
This is before you delve under the hood, where every parameter possible is editable. You want to miss the bottom string on up-strums? There's a setting for that. You want the body to resonate with the strings (very nice on the semi-acoustics)? There's a setting for that. You want to change the tuning? Well, you get the idea.
The only downside is that the learning curve is steep, but that's the price you have to pay for this level of flexibility in the product. The fact that one of the development team contributes and answers questions on the KVR forums helps with this, and he has personally helped me out with a "how do I get it to do this?" type question.
If only they had a version for bass and acoustic guitars, I'd dust of the credit card tomorrow without thinking twice about it...