I've used it for billboards, but as Pete says it's no good for 3D geometry. However, if you want a free billboard tree generator, you're probably better simply using Al Hart's plugin...or Arbaro...which can produce stunning results if you have the means of converting them to something useful. http://arbaro.sourceforge.net/
Unfortunately, for usable 3D trees in SU, there really is no alternative to building them yourself, by hand...albeit by maybe using some kind of modular system involving prefabricated branch parts.