IE6 renders web pages more or less the same as IE4 did - and given it ain't 1997 anymore, that involves an awful lot of dodgy crufty HTML. the only way to design a modern web page these days is to make a proper version, then spend days breaking it in subtle ways such that IE6 can handle it
all well & good, and there are those who swear by IE6's ability to display "all" pages (i.e. all those written using 20th century HTML, or stacks of IE6-specific hacks)
however, then we get IE7. all those millions of IE-specific hacks? they don't work anymore. IE7 almost handles 21st century HTML properly - which means that when it's fed IE6-specific broken junk, it displays said junk as what it is - junk.
this is, of course, a shame - IE7 is vastly improved over IE6 in every way, but because it chokes so badly on the millions of IE6-specific pages out there, is currently a bad choice for primary browser. IE6 is all well & good if you're a firm believer in "let's party like it's 1999", but pretty obsolete for those of us in 2006.
feel free to substitute "opera" or "safari" wherever you see "firefox" in the above