On one hand stability is when software meets a certain standard. On the other hand, it is the ability for an administrator to manage it. You should review the release notes for 6.4 in detail and test 6.4 in your lab so that you do not have any surprises and manage the changes that come with 6.4.
Many users upgrade without reading anything and are upset and disappointed when things change, supposedly without warning. Read the release notes, test 6.4 and determine for yourself if it works in your environment.