I have been peppering eero's "Questions" link for the past few days, and have gotten a lot of information, and have been suitably impressed enough to pre-order.
1. In the Mesh, all units using the same MAC address and Channel. This means no actual "hand-off" needs to be made from the perspective of the client device, even as you move about the covered area. It sounds like Antenna Diversity on Steroids.... There will need to be multi-path-rx consolidation done within the mesh, and some decision making which eero (and which antenna on the specific eero) to send device bound packets to.
2. According to another response I got, however the mesh is currently formed, it's day-to-day operation can be supported both wirelessly or hard-wired (cat5). I am guessing (hoping) this means that a giga-switch could be installed between eeros, and I will be able to use my previously installed pulled cat5 cabling to reach from my Router in the basement to my top AP two floors up.... the guy indicated that wire based mesh will still be faster (at least from a latency perspective) than a fully wireless based mesh. I intend to have a hybrid wired-wireless mesh unless it proves to be too much hype.
3. I did not get a definitive answer (I don't think) about the usefulness of the 2 gigaports on each eero. They are both WAN/LAN auto-sensing, but was hoping to learn that they could both could be used as LAN ports at the same time.... Maybe they can, maybe they can't, but at least one can be a LAN.
4. I have not really received an answer on how "customizable" the QoS settings are. I have a lot of "critical bandwidth" devices (tivos, game consoles, VOIP) and I would like "massagable" QoS settings, by port and by fixed IP or MAC.