From my experience most websites look dreadful.
Many websites have a mobile version but the website doesn't sense the Kindle device type and returns their full screen page. The Kindle tries to compress the page to fit into the screen but in doing so it really messes up the page and for the most part the site becomes unusable.
When browsing a site formatted for mobile devices it is much better and is quite useable.
As far as I know the Kindle is the only major eReader that has a browser.