What are some of the most common scripts which are currently having problems with this update? I'm not sure if it's worth the new features or not... any advice?
Many more popular software packages are now at least compatible with PHP 5, so the main issue you'll run in to is with home-grown applications. However, it is worth it to bite the bullet and try an upgrade to PHP 5, as with PHP 5.2 there are much fewer issues and many more reasons to upgrade, both performance and feature wise. In addition, bugfix PHP 4 support will be dropped at end of 2007, with security fixes ending on 08/08/2008 (see http://www.php.net/), and many projects have committed to only supporting PHP 5 for new releases (see http://www.gophp5.org/). On a production server, the recommended way would likely be to add PHP 5 in addition to PHP 4, and enable it on a script-by-script basis, working through any possible issues as you go. A good document to keep handy would be the PHP 5 migration guide: http://www.php.net/manual/en/migration5.php
You only need to upgrade if you are planning changes to your site. PHP5 has new programming syntax which will impact on how you code. I have upgraded all my sites running a variety of CMS and framework systems and it's been seamless in every instance.
Thanks for the help and advice, I think i'll continue investigating although I don't anticipate any problems.