If I were to pick the feature in beta.17, it would definitely be sealed sending – Whisperfish is a lot more reliable now! (But it’s still in beta!) That was the most critical piece of technical dept (in my opinion) that’s now pretty much paid, so in the future we should be able to focus more on new features and polishing the existing ones instead.
For the bug-hunters out there, the logging can now be enabled and disabled from the settings page (restarting Whisperfish is required) and there’s now log rotation: it keeps the five last logs, so that should protect from “whoops, I lost the log because I restarted Whisperfish” situations.
Beta.17 is definitely something you should upgrade to 