Today we've pushed live the first public beta of Sandvox 2.5.4. Full release notes are here:
The most important thing is the new FTP system, which you can opt into from the preferences ("cURL"). We'd appreciate as many people as possible trying this out the next time they have change changes to publish. The old ("ConnectionKit") FTP system will be going away soon, so any heads-up we can get on possible bugs is appreciated. If you do see an issue, please use the Send Feedback menu in Sandvox to report it.
The beta's available for non-Mac App Store customers; just set your preferences to include checking for betas and 2.5.4 will show up.