As quoted from http://www.valvesoftware.com/:
The site has its own Mac area too: http://store.steampowered.com/browse/mac/Valve is the creator of Steam, the pioneering game platform that distributes and manages over 1,500 games directly to a community of more than 30 million players around the world.
The consoles you mentioned provide a different experience, not necessarily better or worse, it just depends on the person. For example, some people prefer to slob on the couch and play on their huge telly which is hooked up to their surround sound system. Others prefer the added immersion and keyboard + mouse controls PC gaming provides.
It's all about the games for me and I've got no brand/platform loyalty whatsoever. I'll simply buy whichever version I think will suit me best eg. Portal 2 on 360 for local split-screen co-op or Bioshock on PC for the controls & atmosphere. They all have their strengths and as a hardcore gamer...Must have them all (well, apart from the Wii)