Well what do you want to know? I have setup most of my sites to be compliant for certain parts but am not sure how effective it is in improving SEO rankings if that is what you want to know.
W3C provides guidelines for coding that if you comply with, should mean all users of all browers will be able to read your information as it was intended. This is because the W3C guidelines are used (and abused) by manufacturers of web browsers. You dont have to use it and you site may still work. But, it is avisable.
It's a good idea since if your web page is W3C compliant, then it will probably render correctly in most of the browsers.
your web pages will be loaded faster than before. it's somethink related with browsers. i'm also quite sure that google loves the w3 comliance web pages
Nonsense. That is not going to be true in most cases. I'm also quite sure it doesn't do a damn thing for your Google rankings.