When should you use metal studs?
Around here they are generally only used in commercial construction or in buildings where flamable building materials like wood are not allowed.  Wood is just much easier to work with generally.

Some advantages of metal over wood:


They don't burn.
They are always straight.
Easier to run electrical through (come with holes, so no drilling needed).


And wood over metal:


Easier to work with (cut, shape, toe nail, etc).
Stronger.  You can't use metal studs for load bearing walls.
While metal is generally cheaper than wood, the extra accessories needed (special boxes, conduit, etc) can make it more expensive.
Wood is generally easier to get - most of the big home shops don't carry steel studs.