If you are a guitar player and don't already own one of the really high end sims the full version is definitely worth the price. Even if you do own the other ones it is still a very nice program that has features that helps provide much more realistic tones. Two of the nicest things about it is a) the cab/room/IR options and b) the fact it responds to input signals like a real amp would (much more so than something like GuitarRig5 which I also own).
a) If I have a good amp/drive/FX sound going on but it isn't quite punching through how I like or sitting in the mix properly getting at the cab/mic/room settings is almost guaranteed to polish things off. You have something like a dozen types of mics to choose, you have two possible mics on the front of the cab (and they can be different mic models) that you can place anywhere in relation to any of the cones on the cab (not just a few limited choices like in other programs) AND on top of those two mics you can also blend in a 45 degree mic and a rear mic which REALLY helps. You can choose from I think four or five different rooms all of which sound exactly as they should (IMO). There are invert phase switches and high/low cut filters on everything which is really nice. You can also choose from a buttload of included impulse reponse cabinets (although you cannot tweak these like the other cabs but they made sure each style of cab IR has lots of variations representing common mic set ups) and you can also import your own. And all that is only if you just have ONE cabinet in the chain. With the little TH2 mixer you can send out to two distinct cabinets with their own independent settings. Beyond that you could add even more cabs in a chain I think but I'm not sure how much benefit it would be at that point. You can save cab setup presets independently too so instead of creating a whole new bank in the main presets library you just save it in the cabinet dialog.
You do however have access to the mic positioning/blending stuff in the Sonar version but you don't nearly as many cabs and I'm pretty sure the mic selection is limited too. That in itself it's a huge bonus of the upgrade.
b) When I play into GR5 at varying levels of volume/attack/etc the sim just seems to remain the same. There is no character or color differences. The only thing that really happens is the drive or effects won't engage as aggressively if I play softer but it sounds unnatural. In TH2 they have some crazy way of processing the signal so when it's driving the amp models and effects it reacts like you would expect their hardware counterparts to. I personally think that most of a guitar player's tonal character comes directly from their attack/right hand technique and it's a special skill to use that to millk the tone and nuances out of the amp/effects being used. In other sims I've used that reaction/interplay seems to be missing and it sucks. I have to completely alter my right hand technique because those subtle nuances I'm trying to convey don't translate well and just make it sound crappy. With TH2 set up properly I can really get into things because it's doing what I expect it to. I don't have to think about it.
The Sonar version of course still has that really nice processing and the including Overloud custom head works quite well for a lot of stuff but to REALLY hear how good TH2 is at this kind of thing you gotta try the Bogner and Randall heads which only come with the full version. I swear you can almost feel the tubes glowing and reacting to what you are playing. It's unbelievable. I do a lot of ultra high gain stuff which is much easier to deal with within a sim because you are processing/distorting the heck out the signal but I also like doing bluesy/jazzy stuff using clean/slightly tube driven tones. That's where it really becomes apparent how good these models are.
Beyond that though all the stomp boxes and rack effect units that come with it make it super useful for pretty much most hings I'd ever do. I own and have played through a lot of the stuff modeled and it is all pretty spot on. I can tweak it just like I would the real thing so that makes things a lot easier when dialing in tones.
The downsides to me about TH2 are... it's a little overwhelming at times. There is just SO much going on and SO many things that can be tweaked it can be a little hard to sort through it all and nail down a specific sound. Having experience with guitar gear, how it's set up and some recording know how is really imperative to getting the most out of it (unless you don't mind using presets which I only really use as a means to get ideas on how to set things up myself). The other thing is I find that even though the main windows are pretty easy to understand and use (the mapping windows) a lot of stuff is hidden so you really have to give the manual a good once over. The presets section annoys the heck out of me too. I find it's a really weird set up for saving, searching and loading things. Most people really like it though so it could be I'm just a little dumb.
Anyway... I think it's great and have never regretted dropping the extra cash on it. I just wish I had known about it sooner because I upgraded to GR5 literally a month or two before X2 came out with TH2 in it. I haven't opened GR5 in well over a year and even then that was just to screw around with some bass tones (which I still kind of like GR5 for but I intend to buy the Overloud MarkII bass program which looks way batter).
Cheers.