I'm already using SVN 1.5, both sever(VisualSVN 1.5.1) and client-side(TortoiseSVN 1.5.1) but we are using a fairly lengthy SVN Branch-Merge dance.
How ready is the Merge-Tracking facility of SVN 1.5.x and is it yet possible to use it from TortoiseSVN?
Subversion releases are generally pretty solid, version 1.5 has been in the works for 2 years. That being said, there are always some known issues.