TL;DR - Iron 2 is a massive update to what was already my favourite of the Virtual Guitar (VG) plugins. (It is one of four different plugins, covering different styles/genres.) It seems to sound better, it is capable of a lot more variety than the original and, used subtly, it can sound just like a real guitarist.
For those who are unfamiliar with the concept, Iron 2 is a sample-based guitar instrument with two modes. In Instrument mode you can play guitar from a keyboard, just like a synth, albeit with a very narrow range (just two-and-a-half octaves). In Player mode, notes/keys trigger prerecorded riffs and phrases, selected using the lower octaves, with the pitch/chord being selected with the upper octaves. To create the tone you hear, there are multiple "Characters", to simulate different guitars/pick-ups, different amp/cab options with Drive and various effects you can dial in. Each preset stores those settings and matches it to a set of riffs/phrases to suit. To create your own preset, you select a set of riffs/phrases, choose the guitar character you want, dial in your own Amp/Cab/Drive settings and add effects to suit what you're trying to do. It's all pretty simple, really, but the results are mostly brilliant, in either mode.
Iron is the VG instrument for harsher guitar tones - Punk, Grunge, Alternative and Hard Rock - and it delivers on those genres nicely. Iron 2 extends that versatility further than the original in a few ways. The individual Amp/Cab modes are more different from each other than they used to be, so they cover more territory now. There is also an extra mode - Direct - that bypasses the Amp/Cab section altogether, giving you a raw electric guitar sound. A new Damp slider adds even more variety to the sound, as does a Force control, although it's a bit subtle most of the time.
The keyboard layout is consistent and easy to work with. The bottom two octaves always contain the same riffs, the next octave triggers a set of riffs you can change, that suit a particular style/genre, and the upper two-and-a-half octaves are the ones you play in either Player or Instrument mode. There's a Latch button so you can set off a note and play around with the settings, without having to run your host's sequencer, and timing options to play the riffs at half or double speed, as well as a subtle setting to add a bit more human feel to proceedings (Player mode only, of course). Playing a new note legato doesn't retrigger the pattern, which is how you add melody to the single tone/chord riffs. e.g. I select a riff that goes chugga-chugga-chugga-chugga over a bar. I trigger it in A and it plays an A power chord. If I change to a G half-way through, it keeps playing the riff but pitches it down to a G power chord, so I get the first two chuggas in A and the second two in G.
The biggest change is to the effects section. The original Iron had Chorus and Reverb knobs and that was it. Iron 2 has a stomp-box with 16 different effects to choose between, as well as Ujam's great Finisher multi-effects algorithms that can add anything from really full-on Industrial Metal distortion to trippy psychedelic excursions. If you choose the direct out option, you can add a stomp-box overdrive and a chorus plus reverb or something from Finisher to get some very different guitar tones than what you'd get through any of the Amp/Cab simulations.
Are there any downsides? Nah. It does just what you'd expect it to and it does it really, really well. The only reason I haven't given it 5 stars is that the 2.5 octave limit in Instrument mode is just a bit too restrictive, especially when VG Carbon and VB Rowdy offer more. Still, the sample content has tripled in size, so that should give you some idea of how much they have added to this very worthy update.