MARUKO DRINKING TEAM  I panned this software last year (see below), but it is really much better now. The guitar tones and effects have always been great, the problems were always tons of bugs, nonworking features, and insanely intrusive and lame product registration mechanism.

The product registration part still sucks; it is some godawful Flash-based app or something, and it is horribly designed and still atrociously bad. But many many bugs are now fixed or gone in version 3.x, and the experience is much better. The guitar part is the same (very good) and the general software quality has improved a lot (it's now average, no longer terrible).   
(Version 3.1)

ESPIRIDION  My experience is different from that of the previous reviewers. I have not had any problems with the software, the registration method, or with recording.

I'm a fan of amp/effect emulation, and have used both analog and digital emulators.

Sound-wise it is really good, although these are not the best emulations I've heard. I'm definitely not selling my amps.

On a negative note, I was a bit surprised by the jump from 2.3 to 3.0, and the respective upgrade price. This is not the same experience I had with Battery, B4, and Pro-53, for which I received numerous updates and even upgrades free of charge.

I have not used Waves GTR, but maybe I'll check that one instead of upgrading.

By the way, this sort of technology has been around for maybe a decade. The first emulator of this type that I used was Amp Farm. Other digital emulation alternatives include Roland's VG equipment and H&K zenTera amps. The latter are the best I've ever heard.

I highly recommend Orren Merton's "Guitar Rig 2 Power!: The Comprehensive Guide"  
(Version 2.2)

MARUKO DRINKING TEAM  This software does have the best guitar sound you can get from software. But unfortunately, the software is otherwise so ATROCIOUSLY bad that it often does not even function at all. The software registration and activation is HORRIBLE and literally fails to work the majority of times. The software stops working and has severe bugs that break basic functionality. The forums are full of users who recorded songs, applied a minor update to fix some other heinous bugs, and then found their songs were all broken. I was motivated to write this review because right now, I have a brand new MacBook Pro, fresh out of the box, and Guitar Rig's activation just doesn't work. I have probably spent more time trying to make this extremely low-quality software work than actually ever playing guitar through it. This would be great software if it worked, but it is simply buggy garbage. Stay away.  
(Version 2.2)

ANONYMOUS  It works. In ProTools create an AUX Track with GR as Plugin and route the output to one of your audio tracks and record the generated sound from your guitar.

BTW: GR is one of the best things currently available for guitarists...  
(Version 1.1)

ANONYMOUS  There is a problem with this software, you can't record the actual sound, you have to record the dry signal to Pro Tools, Cubase, etc. and then apply the plugin.   
(Version 1.1)

ANONYMOUS  Just downloaded the demo.

Umm....Wow! The Sounds, Tones, everything sounds amazing! It puts my Multi-FX unit to shame. Haven't tried recording with it yet and I'm guessing it will be very CPU intensive.

I would really like to try the foot controller with it, that would make it perfect!

Native Instruments does it again!

:)  
(Version 1.1)