I'm running the T500s up front in stereo and am considering upgrading the center and surround channels to MC500s from older CS speakers (a very thin, excellent sounding center channel that I'd only replace to make my center channel match the 500s, and ensemble surrounds). I've loved the crisp dialogue playback from my thin center channel, but for music playback, surround playback is way too uneven with my current configuration.
Why go MC500s for surrounds? The S300s are great for films, but I'm not convinced that they're the right choice for music. To take full advantage of music surround sound modes and to make surround effects more powerful on movies, I think the MC500s are a better investment. If I go with an HK receiver capable of Logic 7 processing (AVR 745), which I'm inclined to do, or even a Marantz with Circle Surround II processing (SR8100 or SR9600), I would expect this speaker configuration to shine.
The T500s are pretty great for movies and most of the music I listen to. Frankly, I do wish the bass response was smoother in music playback, though I'm finding that my overall listening experience is vastly improved when playing SACDs versus CDs.
Cambridge Soundworks has done a pretty great job of surviving its acquisition by Creative and not losing its commitment to quality products, and unprecedented customer service. The ability to evaluate their speakers and receiver offerings for 45 days with a no questions asked return policy and free shipping to boot, is absolutely beyond belief fantastic. There's also always a knowledgeable guy available to discuss your installation with. Great company! Love'm.