we are running WP 3.0.1 with the Atahualpa Theme and +80 complex plugins.
Worked right out of the 'box', easy install, easy setup, easy configuration - brilliant
Before we had W3 Total Cache installed, which almost 'crashed' our site. We are sure W3 is very sophisticated and can be tweaked by thousands of parameters. Having said that we really enjoyed the ease and simplicity of WP SC. According to our SEO monitors most of our sites score +80/100 for loading time -