I download k'd versions off usenet first so I can test em out without restrictions for a few weeks.
If I like em, and know I'll be using them regularly I'll buy the full version.
Especially VST synths where you can't tell if they're any good unless you use em in a song first.
Like, ReCycle, Absynth, Pro-52, X-Phrase and Plex etc.. I ended up buying full versions, but Virsin Cube I tossed out cause it always crashed.
I also discovered Cubase 32 used up too much processing power on my older Mac so I went out and bought Cubase 24 instead.

Demo version usually have too many restrictions... like AmpliTube was impossible to test out without the full version. But after using it in a few songs it was worth buying the full version.