I recently bought a full license for circle. I'm pretty happy with it overall.
There are certain patches (maybe like 20 total) that as soon as they are selected, will make a click, then a whole crapton of noise, as the CPU climbs to the top (even with no notes being pressed).
My setup:
Intel core2 quad, running on windowsxp, 2gig ram.
Sequencer host: renoise
If it will help I can go through and document each patch that has this behavior.