DSSI is a programming API, not an application, so screenshots are of limited use. But here are a few shots of some of the existing plugins, so as to give an impression of what it's all for.
Here are four of the plugins available from the DSSI download page. This screenshot shows the native GUIs supplied with the plugins, running in the DSSI reference host.
The plugins shown are (left to right) Xsynth, the hexter DX7 modeling plugin, and (above and below) the simple stereo sampler and example polyphonic synth included in the DSSI examples, together with a qjackctl window showing their audio output connections. To run the host with this set of plugins, I typed:
jack-dssi-host xsynth-dssi.so trivial_sampler.so:stereo_sampler hexter.so less_trivial_synth.so
As you see, the existing plugins have fairly boring GUIs (they were written by boring programmers, after all), but even so, any dedicated GUI is usually easier to use than the generic plugin GUIs found in LADSPA host applications. For example, here is an Xsynth plugin hosted in Rosegarden, showing the Rosegarden plugin GUI on the left and the Xsynth native GUI on the right:
Rosegarden has the tastier widgets, but the fact that the plugin's own GUI knows how to group and organise its widgets makes it much easier to use. (Of course these windows are synchronised -- as you edit values in one, the other updates itself as well.)
Here's a handful of plugins being hosted by Rosegarden, including one Xsynth, one Fluidsynth plugin (with the Vintage Dreams Waves soundfont loaded), and a freeware VST plugin (Cheeze Machine) using the dssi-vst wrapper:
This is Om hosting ll-scope:
And finally, ghostess hosting instances of WhySynth, Sineshaper, and xy-controller-dssi (the last being just for show, since Om is currently the only host which can do anything useful with xy-controller-dssi's control outputs.)