While we are developing TeamCity 7.0, some important fixes are being back-ported to 6.5.x branch to deliver the fixes to you earlier then in 7.0 release.
Latest branch snapshot is released as 6.5.5 today and is available from the official download page.
Update is recommended for all users as there were some performance improvements. See the full list of fixed issues.
Data format is the same as in all 6.5.x releases, so if you are already on 6.5.x, it is possible to downgrade if necessary. Anyway, creating the backup before the upgrade is highly recommended.