That said, truth: just talking about the "big two" commercial sims, I haven't used any IRs with Amplitube 3 or with Guitar Rig 4. Their built-in cabs (the comprehensive cab/mic/room system makeover in AT3, and the Control Room) are always great for my needs. Before Amplitube Fender I often used IRs in my own recordings, though for the sake of reviews I'd never do anything that wasn't part of the software itself (should be obvious, but just in case it's not clear). Now, I really don't, even though I have the full Recabinet 2.0 pack which competes quite well for sound with the RedWire IRs. I just don't have to go outside of the software to get comparable sound quality.
In the case of Guitar Rig 4, the Control Room thing is basically a collection of quality IRs that are all in-phase and sound good; getting a good result with it is, as a result, quite easy to do. The IRs that make up the various cabs are high-end signal chains captured very well, and you have ample control over them as far as position in the mix goes and relative volume. Great module, one of the big improvements in GR4 over GR3 (though I did some testing, oh, eight months ago now that showed GR4's Matched Cabinets were improved substantially over GR3's as well).
In the case of Amplitube 3, I just love the new cab setup. It works very well, it's very intuitive, once you become accustomed to its work-flow you should find it very easy to get a good sound. Of all the sims I've used, so far it is the closest to actually miking up a cab - if you know how to use X and Y microphones to get a good sound out of a given speaker cabinet, that knowledge translates better to Amplitube 3's cabinet modeling than anything else out there except maybe Softube's Metal Amp Room (which is limited in that it doesn't give you a variety of microphones to pick from). My opinion, my experience, my ears, my gear - my story and I'm sticking to it.
I like the idea of Vandal's physical modeling for the cabinet. It's a notion which I think has a lot of potential. Just as there's nothing inherently magical about the behavior of a circuit involving vacuum tubes and other parts, there's nothing magical about the behavior of the amp's output electricity into the cabinet, the movement and output of the speaker as it vibrates the air, the effect that has on the microphone, the microphone's own diaphragm (of whatever construction)... All of that can be accounted for mathematically, and thus modeled accurately. I regret that have not had the opportunity to try Vandal to see how well it pulls off the concept, because it's a cool concept. Its realism depends on how well the phenomena are understood and modeled, and what generalizations have then to be made in order to optimize the processing speed for real-time use.
Oh, one more thing, call me crazy but I'm not impressed with the various Nebula cab models around. They don't really sound any better than IRs to me, but they sure do COST a lot more in terms of processing power (and IRs are already relatively hungry for realtime use, depending on the number of taps...). Advocates say that it FEELS better. Well, feel is subjective, you know, I can't judge that as easily as I can judge sound. But then sound is subjective too.
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