I got my GuruPlug Plus a week ago. I am using it with one Ethernet cable connected, Wifi sometimes on, sometimes off. The Ethernet is connected to a Fritz Box (100 Mbit).
a short update: I wanted to test the behaviour with Gigabit Ethernet. Since I do no have a Gigabit Switch I connected directly to my MacBook. And in addition I used the JTAG console to be able to observe what's going on. The Guru booted fine. As expected it had no IP address (no DHCP server). I configured it by hand and got the connection to the MacBook. The GuruPlug did never crash nor reboot nor anything suspicous over several hours. But under load it several times lost the IP address and I had to manually do ifconfig to set it again. Which always worked. So there is something fishy with the Gigabit Ethernet. Could also be software related, though.