In a couple of ways, this is an interesting forum. One is that there's almost no mention of modeling/modelers/modeling guitars, etc., and no separate forum for any of that. Even MLP, that bastion of tradition, now has a separate area for modeling. And the attitudes and comments coming in about modeling in general sound on this board sound as if they were from 2005. That's not meant to be insulting; it's just a bit surprising.
I've got 15 tube amps between what I have in storage and what I've got in the den, and I love 'em to death. But I've also got an older Axe FX Ultra, a borrowed Helix (I'll soon have my own), and some HD and HD500 gear, some Pod XTs and a Bass Pod XT. I've been using the HD500 with a Two-Notes Torpedo C.A.B. (turn off the cabs on the HD500 and use the C.A.B.'s IRs, and the C.A.B. also has some amazing sims of tube power amps, including EL84, EL34, KT88, 6L6 and the like.
The use of IRs has changed a lot more than just amps and cabs -- Pete Thorn pulled a (free) IR of a Taylor 314ce being miked with a Shoeps off the internet and ran his Takamine (which has a good piezo setup) through the Helix with the IR in place, set to 100% wet, and what comes out the other end is an acoustic, miked in air. He's been using it when working live with Chris Cornell, and it's stunningly good (needless to say, I hunted the IR down, bolted it in and tried it with my old Variax Acoustic 700, and it was amazing).
jp_nyc is pretty much correct that a lot of Line 6's amp models are based on older amps -- they actually have the amps used sitting on display in the back in Calabasas. If you like, they'll fire them up and you can listen to the actual amp compared to the model, and Line 6 has done a pretty good job with them. Same deal with the guitars used for the Variax models. There's really no reason to add or swap out the current set for new ones; we're at a point where, with a lot of the current modelers, you can profile any amp you like and plug that in. But with a lot of them, modern amps are modifications of circuits found in even older amps. And so you can make modifications of the older models on the XT with the editors available and get pretty much the same thing. There doesn't seem to be anyone here chiming in so far who has experience using the various editors on the current modelers. A lot of the "complaints" are about things that are easily tweaked and saved to the units.
The fact is, the models have become SO good on the new modelers that I think we're mostly going to see advances coming in the realm of the user interface on these units. Here, the Helix is leading the way, with a really big screen usable for any kind of editing you need to do, routing options that encompass almost anything, two-stage switching that includes contextual color surrounds, contextually responsive LED "scribble strips," and multiple sounds/scenes within a single preset -- no delay or loss of time-based "tails" while one preset is moved out of a DSP and another one loaded up. There are four FX loops on the Helix. There's the ability to output to mixer and local monitors differentially. So much more.
I'm originally a keyboard player; a Hammond B3, Fender Rhodes 88 and (when I could get it) a grand piano. Over time, keyboard had its own "modeling wars" and there were those who said no electronic gizmo could ever fill in for a Real Grand Piano. My current Korg Kronos will not only let me choose the nationality (Japanese, German, etc.) of the grand piano I'm going to play, but will also allow me to choose how far I'd like the lid open. Those wars are long gone and people are now writing and performing whole movie scores on a single keyboard (oh yeah, mine also has a 16-track sequencer built in). We're WAY down that road with modelers for guitars. There will always be those who argue over whether some tiny corksniffing intangible is *quite* right in the oboe tracks, or if there's the sound of spit dripping in the right kind of wood bore that no one would ever hear unless they had golden ears. But that wrestling match is over, and people are now simply being creative.