I found Bryce 5.5 to be as stable as 5.0, but 6.0/6.1 to be troublesome.
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When I got b55, I kept both b5 and b55 installed. When I got B6, I uninstalled b5 and kept b55 and b6(now 6.1) installed. By my measurement, B5.5c is the most stable build. Not perfect. But pretty good. 6.1 is faster in large part because it supports multi-core/multi-processors. So if you have a two cpus and both are dual cores, Bryce 5 will be able to make 100% use of 1 core. But with 6.1, it can make use of betweeb 70-95% (varies based on seemingly random factors) of all 4 cores.
B55 has some OpenGL views which really slow your system down, but makes for a better preview of what you are doing. It is especially helpful when doing booleans. I used to toggle between that and wireframe. Pure wires is always faster. But, the OpenGL view is the only way short of clicking the render button to sometimes figure out if you have stuff lined up. With Truespace or Cinema4d, I use Deep Exploration for this preview. With Lightwave, I do not need DE as LW has such a good OpenGL engine built in.