It depends...

Many musicians (Me too) are using it in Live condition.

as you can see here
http://www.varranger.fr/vforum/index.php?action=dlattach;topic=124.0;attach=31;image

or here
http://www.varranger.fr/Gallery.php
http://www.youtube.com/watch?v=NUQH4WSQjgA
http://www.youtube.com/user/varranger#p/a/f/0/Nqt5bjswOVg
http://www.youtube.com/watch?v=0amhVeE28pQ&feature=related
...

There is many controlling options.

Of course, a yamaha keyboard with a lot of buttons like your s990 is very good too.

The 2 systems have their own interest.

I personnaly use my Ketron SD1 on some gigs, and love to take my vArranger + SD2, on some other gigs.
Depend on the context...

Dan