Will I be able to migrate my data over to 0.6 when it's released?
Yes, dedicated tool will be provided that will let you migrate your data from 0.5 installation to 0.6 once 0.6 is released.
How stable is 0.5?
0.5 line is stablest codebase around, but, that doesnt mean its stable. There are few known bugs like file upload from quick reply not working or unability to remove thread prefixes using threads list moderation tool or weird data taxonomy causing Google Bot to rank user profiles higher than threads, but there are no known data loss bugs. Misago 0.5 already saw action on very active forums with 100 active users at single time and fared very well under such conditions.
When will 0.6 ship?
When its done (tm). Superoptimistic prognosis is somewhere in 2016, but real ship date depends on speed of progress that will be made.