I have a beat programmed in FL Studio 6's VSTi, a bass line programmed in another VST instrument (Muon Software's CM-303, distributed with the UK magazine Computer Music) and KarmaFX Synth on pads. The problem is that the moment I press Play in SONAR with this particular combination of VST instruments, KarmaFX Synth stops producing any sound, and stays dead until I exit SONAR. The bassline and drums play fine (and both accept real-time MIDI input perfectly too), but Karma Synth's programmed MIDI part does not play at all, nor does Karma Synth want to accept any real-time MIDI input. None of the displays on Karma Synth show any movement, with either real-time MIDI input, or with trying to play back the programmed part, which leads to me thinking that there's something in KarmaFX's code that's making the sound engine self-destruct somehow.
If I restart SONAR and load in my project again, I can play Karma Synth fine via real-time input. It's only once I press Play that the synth dies completely. Deleting and reloading the Karma Synth VSTi does not help.
To make sure that this bug was not with the FL Studio VSTi, I tried it with two versions of the VSTi - the one that came with FL Studio 6.08, and the one that comes with the FL Studio 6.1 public beta 1. There was no change in Karma Synth's behaviour with either version of FL Studio's VSTi.
This bug is reproduceable, easily. Just load in FL Studio VSTi into SONAR and program a beat. Open up Karma Synth in SONAR and program a part. Hit Play in SONAR and Karma Synth will die. Incidentally, Karma Synth works fine when I take FL Studio out of the equation, and of course, as I said before, FL Studio works fine with other VST instruments. It's only when FL Studio and Karma Synth are together in the same project that Karma Synth does not want to co-operate.
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Incidentally, KarmaFX Synth does not properly respond to SONAR's panic button either. The panic button in SONAR is meant to stop stuck notes, or reset rogue controller data - but with KarmaFX Synth, SONAR's panic button actually causes stuck notes.
Despite these bugs, I'm really loving KarmaFX Synth. To be able to create anything you want, regardless of complexity, for free, is a dream come true. Also, since I switched to SONAR 5 from Logic 5.5, I'd really been missing Logic's ES2 synth. Well, until I started playing with KarmaFX Synth, of course.
Seriously, KarmaFX Synth is the closest-sounding synth to Logic's ES2 I've heard in a long time. And because it's modular, it's actually better! 
Thanks for such a great free synth!