I think TH2 (the version in Sonar producer is what I've heard) sounds about as good as Guitar Rig 5, but isn't as quick to use, and I really like several of the effects that Native Instruments makes for Guitar Rig.
 
The sonically best guitar amp software I have used is Magix / Samplitude's Vandal - I think it's really a cut above Guitar Rig.
 
To the original question of whether a plugin is worth 80 bucks, I would say that all of the very best things in life can be had for free, so nothing is worth 80 bucks.  Someone who needs to do a task for which their current toolset is inadequate, however, may have to acquire new tools in order to move forward.  The labor required to make those tools is only one of the factors which influence its market price.

BBFG#
A question for those that own it.
How do you think it compares with Guitar Rig?
I have it, and Guitar Rig sounds kind of sterile to me, even when it's suppose to be simulating tubes. I bought IK multimedia's Total WorkStation3 because the price was so cheap in getting everything including their effects and Amplitube 3, but so far, all the effects and Amplitube crash every time I try to open it.
So this seem like it would be nice and seemless to integrate into Sonar X3 (and I'm assuming as a stand-alone too?)

So how would you rate TH2 full with those other two?

BB...I use amplitude 3 all three time with no issues...if ur running 64 bit, make sure u've updated to 64 bit amplitude...
 
Also, add a Sig in ur user profile that includes ur system specs... 
 
I AGREE That $79 is a very fair price for the depth of the rig...it's very nice... this isn't an iphone/android marketplace!
 
Equals amplitude in my book, with each having sweet spots. GR4 is thin to me but luv it for bass /guitar effects...all r great... and sculptable.
 
If I were u, if get amplitude working first... no reason it shouldn't!
 
 

I've had the good fortune to work with just about every amp sim due to all the reviews I wrote over the years. I find they're like real amps - they're all different. They also all have different things they do well and things they don't do well. For example, Amplitube does excellent clean-to-distorted transitions; Guitar Rig has better modulation options than anyone else; Scuffham sounds good but there's not a lot of variety; the Waves cabs are excellent; Eleven sounds harsh until you use the parametric stage to dial out the harshness; Line 6 POD Farm, despite being the oldest of the lot, has some sounds others can't get; Digital Performer has some really good guitar effects bundled with it; SofTube's amps have their own character you either love or hate; Peavey's ReValver has the most programming flexibility by far (down to the component level!); and so on. I haven't really found the high and low points for TH2 yet but I'm sure it too has features others don't have and lacks features others do have.
 
I often put sims in series, e.g. one with effects, one with the amp, and one with the cab.

Disclaimer: I'm not really a very proficient guitar player and only own one guitar (Blade RH4).

TH2 sounds different than Guitar Rig to me and as sort of a general rule I really like the high gain settings over Guitar Rig. The branded Randall and Brunetti models sound really nice. Guitar Rig on the other hand seems to have more versatile fx. I think they complement each other well that way, and I'm not disappointed with spending my 79 bucks for the TH2 upgrade in December. I didn't specifically buy Guitar Rig but I had used it before and it came with Komplete 9 which I also bought in December. Again, they do different things well.

In a google quest for the ultimate amp sim Scuffham S-Gear came up and after trying it out I had to have it. It's by far the most limited of them all but it sounds very inspiring to me and when I turn up the volume a bit, it feels like playing through an amp. I don't usually get that feeling from the others, although TH2's high gain Randall is close. Scuffham is "only" 99 bucks. There are not very many options but they all sound great, and not unimportantly, the presets are instantly playable (for me in direct opposition to Guitar Rig).

All of it purely my opinion. Scuffham has a fully functional demo. TH2 is already kind of its own demo inside Sonar. So try them all and go with what seems to suit you best.

I may get this just for the price, which is good btw.
 
I like scuffham best of all.