Renewal is free, but you may pay a fee in some cases (like bitcoin transactions). You own it while you renew before expire time and you are the only one to be able to control it !
It's early days still. So people will need to adjust their settings to use nameservers with .bit support. Sometime in the future .bit may be added as an official TLD... but who knows
I hope it well never become an ICANN TLD, otherwise, someone may have control on this TLD and this is against our aim.