Before I go though the hassle of doing this: is it useful to do? This feature allows your site to display URLs that are easier for search engines to index. Normally, the URLs of the site look something like this: http://www.yoursite.com/comments.php?id=1_0_1_0_C With the URL rewriting feature enabled, URLs will instead look like this: http://www.yoursite.com/comments/1_0_1_0_C/ This fools search engines into thinking that they are accessing a directory index rather than dynamic content, which greatly increasing the chance that your site will be thoroughly indexed.
I recently redid my site to use modrewrite. If you have a well designed site that uses templates effectively and a good database backend, changing to a modrewrite style shouldn't be too difficult. I switched www.bbconnector.com over and all I really had to do was update a 6-12 pages and all was good.
There are I think at least two other threads on this question at DP. You really don't need mod_rewrite and there are some downsides to using it. See the HTML and phpBB-vBulletin forums.
Yeah, one of those downsides is having too many rewrites can bring a server down! My site started as a PHPNuke installation - when I got savvy to SE-friendliness I installed the google tap module which mod rewrites every area of the site, well guess what, nuke runs every page off of one URL - modules.php or gate.html after it was rewritten. As my traffic increased, the server load on my VPS was huge thanks to that .htaccess file. I've since redone each site area into little standalone programs with their own .htaccess and it's much smoother. It's been good practice and I've learned alot of PHP and Mod rewrite skills in the process...scary for a while though, it was like my site was about to go down at any minute.
Another is creation of duplicate content. The bottom line is (1) you don't need it and (2) in most cases you are better off not doing it.
I also don't think its needed, I just got 12,000 pages listed within a few weeks all of them having a ? in the URL. One thing I always try and do is steer clear of page.php3?id=79 and change page.php3?keyword=79. I find this works well and gets another keyword in the url, only use it if it is sensible to do so though.
Options +Indexes Options +FollowSymlinks RewriteEngine on RewriteBase / RewriteRule ^comments/([^.]+)/$ comments.php?id=$1 [L] But if you got pages already indexed, don't do it!! Duplicate content!!!!