They can be quite interesting for sound design purposes, where you use the soundfont as a starting point but transform it significantly with effects and so forth. They vary from awful to fairly good, but even the best ones rarely get close to the sound of whatever instrument they claim to be. I have a collection on my computer of about 1000 free soundfonts.
What version of Ardour are you using and where did you get it from? This offers a “full range” of sounds as defined by the General MIDI standard, which is say adequate but never great.
With the correct version of Ardour, the default synth is a General MIDI synth based on fluidsynth (a soundfont (SF2) player).
