The software reached the servers today, and the Reply tool was accidentally visible on the four target wikis for about two hours. Several people used it, nobody complained about problems or lost comments, and it's been (correctly) hidden again until the team can do some more testing in situ. If you want the code to do a little testing yourself, please ask here.
Topic on Talk:Talk pages project/Replying/Flow
Well done 
I just noticed one problem: the pages I reply on get out from my watchlist. (At first I thought I just accidentally hit the watch star, but I couldn’t have done by accident it twice.) I hope this will be easy to fix, or maybe it’s already being fixed.