Because of the complications (i.e. excessive memory and cpu usage) of using flash and Opera together on my windows (work) system, I've gone as far as totally removing flash from my system. The plus side is that my browser works faster than it ever has; the down side is that I cannot view any flash content. Overall, the advantages outweigh the disadvantages for the time being. However, I would be willing to sub-par flash performance (i.e. gnash) than none at all, as long as my resource usage doesn't spike super high.
In other words, gnash with sub-par performance is still a lot better in my book than flash with ridiculous resource usage.... but that's just my preference. (now, if I could just get an up-to-date gnash for w32)