I use cthulu on a few of my projects and I like it alot, and I haven't yet taken it to its full potential. It has been rock solid in X3e, even with multiple instances. I haven't used it in platinum yet.
Setting it up involves adding a midi and an audio tracks for cthulu. The midi track is for feeding triggers or chords into Cthulu ( you can assign complex chords to single midi trigger notes, or just feed fully fingered chords). The audio track is needed so that Cthulu can derive timing, and to route Cthulu's sine wave based voice for troubleshooting.
You then create a midi and audio tracks for your softsynth. The softsynth's midi tracks input will need to be set to Cthulu's midi output, which will show up as an input device. You also need to click the input echo button on the softsynth's midi track to permanently enable it. If you don't hear output, this is most likely the culprit.
You can easily record Cthulu's midi output by simply hitting the record buttton on the softsynth's midi track. You can also send prerecorded chord to Cthulu by putting a midi clip into Cthulu's mdi track.
I find it intuitive to use, but the more time you spend with it and the manual the more you can get out of it.
Edit: I've started a new project and began using it with platinum. No problems so far.