Most of the 'free' stuff is pretty awful. Like, cheesy 80's sounding synths.
Even some of the 'good' synths are pretty cheesy sounding too... a lot of them are virtual copies of 80's synths.
Personally I think you'd be better off with a decent sampler like Halion or Reactor. At least that way you'll get realistic sounds like piano and strings, plus loops and drum kits.