The only people who have any length of experience will be the inventors and their testers, as they only started shipping to the beekeeping public in the second half of this year.
By "is heavy", you mean the hive is heavy, or the supers of honey are heavy? I ask because I actually think that western red cedar is significantly lighter than pine, so the whole hive is actually lighter than most hives. However, honey is heavy, of course, and if you have 6 full flow frames of honey, you will have about 40lb of honey adding to the weight. If you mean "is heavy" as in the expense, well yes, but this is an innovation, and we are early adopters paying the premium for that privilege!
Now to your curiosity "as to how well the honey flows out". Cedar Anderson has made a great youtube video of him draining a frame in his honey house/workshop. It looks as if it takes about 20 mins to drain the whole 3kg harvest, but the video is edited and takes less than 10 mins to watch it.
Of course the flow speed is going to depend on the viscosity of your bees' honey - thicker honey will naturally take longer, runny honey (like clover and acacia) should be pretty quick. I am very curious as to whether heather honey will drain at all, given its thixotropic nature - maybe "jiggling" the key to shake the cells will liquify the honey to help it flow. We will have to see!
Anyhow, hope that helps. This time next year, you will have thousands of people with a year of experience of the Flow Hive to answer your questions.