Depends on how you plan to use it.
I have a GS3.
Wanted it over the GS2 for the powersoak + headphone amp, and price of course.
I don't use the powersoak or headphones as much as I had planned, since I want to mix against a backing track but the headphone amp does help me hear my mp-1 when I want that feature. The main out is pretty weak when I run it through my computer and mix with a backing track (using the stock HP mixer).
The GS-3 is significantly better than the MP-1 straight to the board, but of course nowhere near as good as a real speaker cab.
I would put it at 75% of a real speaker cab at best. The 25% comes from the filters removing some of the articulation.
Feedback/gain is pretty good....not sure how they get it to do this but I am very picky old school and really prefer to feel and react to the sound and air pressure from the speakers.
For reference, I play live through an Iso cab, and would say an Iso cab is 90% vs having the real stack behind you where you control what people hear and the vibrations circuit through your pickups. The 10% comes from the mixer guys opinions on what it should sound like based on whatever experience they have, and it is probably not the same sound as you want.
So this means if you play live through a simulator, you should expect less than 75% of a real cabinet unless you are also the mixer guy. Things sound very different when you hear yourself on the internet post mix so it is alot of trial and error.
I haven't tried other options...