I just got one of these recently and have my first mead finishing up in it right now. Have to check the gravity to see if it's ready to transfer, but if not today then soon.
I made a 5 gallon traditional, with Linden/Basswood honey and D47 fermenting at 18C. There was absolutely no issue of the thing overflowing, despite a fair amount of foam getting whipped up by stirring. The only thing that might be an issue is if you want to mix your honey and water in this. The pipe that leads to the ball valve can get in your way. True, it can be turned so it faces straight up and only sticks out an inch or so from the side. You have to be careful even so when stirring to make sure you don't whack into it and potentially knock it out. You also have to remember to lower it back down so it doesn't fill with yeast by the end of the ferment. Having said that, I'm starting to wonder if that tube is actually needed. It might be possible to do without it altogether, which would eliminate that worry.
I haven't yet racked from it, but from its construction I'd say it isn't really designed to get the last of the last of the mead off the top of the lees. With just normal racking you will leave about 1 liter of liquid at the bottom. I don't think I usually have 1 liter of lees to leave behind, though I haven't actually measured that before so I could be wrong.
Compared to a plastic fermentor this thing is a little heavier. It weighs a bit more than an empty Corny keg. It's light enough that it can still be lifted into and out of a top-loading fermentation chamber, but I did notice the extra weight. If you're used to using glass carboys, on the other hand, this will probably feel pretty light.
The earlier models had issues with the lid -- the hole in the lid would cut through the bung, and the silicone seal wouldn't reliably stay in place. Both of these issues have been solved in the current model (purchased from MoreBeer about a month ago). This is taller than a typical plastic bucket fermentor, but about the same width.
I can't yet give any feedback on the quality of the mead this produces compared to using a plastic fermentor, but in general I'm happy with it so far.