I recently gave Rosegarden a try, and overall I think it's an absolutely stellar piece of software, except for the lack of one key feature: The ability to automate DSSI parameters in the main arrangement view. Unfortunately, myself (and surely others), would find it exponentially more difficult to make synthesizer music without the ability to modulate parameters over time. I did find the other options to edit a select few controllers for each MIDI clip in the Matrix editor, or manually entering them in the Event editor, but that's just not practical to do.
Qtractor has a reasonable implementation of the type of automation I'm referring to, but unfortunately, Qtractor isn't nearly as polished or stable as Rosegarden. Here's a screenshot showing some of the automation envelopes: