We have completed our upgrade from SQL 2012 SP1 to SQL 2014 CTP 2 on both clusters. All servers are operating normally. We will keep a close eye on them over the weekend and as we pick up peak load during the week.
The total upgrade process took approximately 2 hours (including OS updates and time for health checks). The total external downtime for each service was between 10 and 20 seconds for the two 5-10 second failovers.
We did experience one unexpected issue during the upgrade (a non-critical, a pre-install check) and have informed Microsoft of the issue, they’re working on a fix.