I know that there is some benifit from a seo point of view to rewriting urls. But what are the negatives to doing it. does it have a effect on server performance etc. Do the pros out weigh the cons. I've been doing some reserch on this and found a blog from google where they says its not needed anymore. Is this something I should be doing? I see that this forum dosn't rewrite the urls and it does well in the serps.
It still helps. I would recommend it. There are no negatives unless you are changing over a big site that is already indexed and don't do redirects.
It helps, and there isn't really a downside to using mod_rewrite for a blog or forum. The extra processing to rewrite the URL is minimal.
The pros? Better search engine optimization, by having page titles in your urls rather than somescript.php?id=13043.