Microsoft has recently released the Windows Vista Service Pack 2 RTM, with plans to release the final version sometime soon. Major new features include: Windows Search 4.0, native Blu-ray recording capability, and simplified Wi-Fi connecting using Windows Connect Now.
If you choose to get the RTM, I would recommend, as normal, that you don’t install it on a system on which you depend. This is not the final edition of the software, so there is a good bit of risk involved in installing it. For the general public, just wait for the final release.
Service packs are always major updates. It is always important to install updates as soon as they come out. They include the latest security patches, software updates, etc. Your system will be the most secure if you have them.