Flock is built on the Mozilla code ... it has only been released as a developer preview. So to answer your question if it's better than Opera or FF - It'd be a simple No. The Flock FAQ can be read here
Flock has been released as a beta (0.71 - English Edition or 0.7.2 Photobucket Edition) Released around June 30th Download Release Notes Known Issues Extensions\ Screenshot below
I ve been using Flock for a week. Besides sudden crashes after 2-3 hours, it is perfect. I recommend you to take tour.