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?
It is productive. You can see the merged revision in the Log-Dialog and the reintegrate merge works fine.
However, there is no graphical representation of the merges inside revision graph and according to stephan Küng there will be no merge info in revision graph due to performance reasons.
The merge algorithm will change in future releases to catch some edge-cases better, but it is working real fine.
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.