Just installed Sierra on an external SSD in Cadi usb3 ( EVO 500 Samsung) - idea being to swap with the existing internal drive later (if all goes well) and no need to have a "back up" or to migrate later - just swap the drives. Install on a mid 2012 15" MBP - external EVO SSD 500 (Samsung) - and migrated the El Cap system and existing apps to the new external SSD (1.5 hours) - so far so great! All good so far - going down the list of applications - top to bottom - opening each one, testing and accessing data etc. Was worried about upgrading and the many negative comments on the internet, and hey good to know - but WOW - so far all is awesome - no major issues! Love some of the new features - for example did not know you could play a video in the imessage without going to an external app? - very cool, and of course - very cool to see siri work on the MacOS Have tried Mail - all seemd to be fine - no issues - saw other comments to the contrary - I only have 2 accounts on mail (work emails are separate - exchange server - outlook ) - personal email on mac mail / icloud and GMX.com and both work fine and do not create an "on my computer folder". Had a few apps that still need the legacy version of Java SE 6 - iAccounts / iBackup, Arduino and another app that I can't remember. Pretty solid beta in my view - glad I have it on an external drive - but my feeling is I will swap it with the internal drive within a week or 2 if I don't see any major bugs / problems ( internal drive will become the back up and new SSD will become the Sierra upgrade - daily driver. Very impressed!