If you are running Joomla 1.5 you should consider upgrading to a more recent version of Joomla. This is because later versions are more secure, have access to better features and functionality and more 3rd party addons.
As long as you have a fully patched Joomla installation with current supported 3rd party addons, your site will be OK. However with the upgrading of technology used by Joomla PHP and MySQL, more issues are being found inparticular with 3rd party addons and in some cases specific recoding is required to ensure sites run well. Rather than let this drag on it is better to upgrade your site and enhance it's functionality.
Update Jan 2020 - It is not longer an option to run a Joomla 1.5 site - you must upgrade.