I've been using S Gear for just on 5 years now. I also have software amps from Apple(main stage), Plugin Alliance, Overloud, IK, Kazrog, Positive Grid, Studio Devil and a number more. NOTE. I do not play metal style guitar. I do almost everything else: jazz, blues, rock, punk, indie, funk, 60's, new wave, latin but I not shredding and so my opinion is useless if that's your bag.
I use S Gear in my studio and as yet have not used it live but i am very keen to try it for performance. I just can't quite get comfortable with the idea of a laptop being my amp. if it crashes, I'm dead in the water.
That being said, here are my high level considerations for anyone looking at S gear vs other software modelers.
1. Sound - S Gear is consistently the best and most natural sounding plugin I have. hands down. I have a bunch of others from a wide range of companies but none of them have the reactive and nuanced feel of S Gear. It not an amp but it's damn close and damn close across a wide range of amp types. It responds to my guitar volume and tone in a very musical way. It's easy to forget you are using software, which is the highest praise I can give any amp plugin.
2. Ease of use. it's so easy to get a good sound from S Gear which is crucial for me. When I am writing I don't want to dick around for ever to get something that sounds vibey enough to help inspire the song or idea I'm working on. If I have start knobbing around I'll lose the spark. The presets sound superb and cover so much ground it's ludicrous.
3. Tweakability - If I am in the mood to dig in and start creating my own preset for something, I can get there pretty quick. The UI is not perfect but given the small team creating this product I'm fine giving S gear some grace in this regard. The skeuomorphic approach works well on most things and all the controls do something audibly to the sound when you adjust them which makes learning the UI and dialing in a sound quick. The IR's and effects area is a little cloudy and not as intuitive. And preset management pretty much sucks.
4. Developer responsiveness - Outstanding for me, every time over the last 5 years I have a question or issue, Mike has been back at me super quick and not just robotic answers. He really engages with the questions and has been open and honest.
5. MIDI implementation and interoperability - I use a range of DAW's (Live, Reason, Pro Tools, Reaper, Tracktion plus some others) and S gear is rock solid in all of them. I also use some MIDI controllers to switch patches and parameters, (Akai, Sonoma, Arturia, Novation etc) again S Gear is fast and clean with them and easy to configure.
Clearly I'm a fan but I think this is because S Gear has consistently delivered the good sonically above all else.
Heres what I want from S gear or what I think it lacks.
1. Faster pace of upgrades - While what is does now is awesome in my book, it is long time between upgrades and often they are fairly superficial. I would like to see some new amps or re-voicing of existing amps. the effects and IR area also could use some love. There are also a number of other options that would make the product more flexible but whether due to bandwidth, technical debt or both, significant updates don't drop that often.
2. A live performance mode - not much to say here. the standalone app is close but needs more work to be a solid live option.
It's late and running out of steam. :) I'll add or refine existing points as I think of them.