if you want mainstage, but dont own an apple, this is what comes closest..
However, Mainstage comes with a huge package of instruments, and costs only €30, and not just instruments but instruments of high end quallity...
The main idea of this campaign is that they want to make you believe its less processor dependant then mainstage... but then, who cares on my 2011 macook pro, i have never run out of cpu nor memmory using Mainstage...
Mainstage does everything Gig performer does and more... on top of that, gig performer requires you to preprogram absolutely everything, where mainstage also works for sit and play.
The whole advertisement is a little misleading... but this is a good VST host if it fits your workflow, but definately not something that just started out with VSTs would be looking for. For more advanced users, you could use Gig performer, but also ableton live or Reason as an allaround keyboard host...