The switch part of the adaptor pair is no larger than a normal four way switch, which is good. The four ports at the back are labelled with different colours, as the device supports Quality of Service, with bright green showing the highest priority port, dark green the next highest and the two orange ports the lowest priority. The idea is that you can ensure traffic on one device in your AV setup is given priority, but I'm not sure how useful that really is. I rarely download demos on my Xbox 360 while also playing Uncharted 2 multiplayer on my PS3, but maybe that's just me. The built-in encryption may prove more useful if you're particularly worried about others using their own adaptors to sneak access your network.
Also useful is the presence on both adaptor units of a connection strength LED. This lights up in red if the connection between the two is less than 50Mbps, orange if it sits between 50Mbps and 80Mbps and green if it's greater than 80Mbps. That doesn't definitely mean you'll get transfer speeds in that region, but it's a good quick indicator of whether you should choose a different socket to plug into.
In my home I was able to get a maximum transfer speed of 64.8Mbps, despite a green LED, which doesn't sound great. That is, however, much better than the 25.6Mbps I was getting previously, with a set claiming an 85Mbps maximum transfer speed. To the maths-averse, I should point out that's a better improvement than you would expect just from the increase in the max (theoretically) possible transfer speed. It's also enough bandwidth to stream a Blu-ray movie, including its Dolby TrueHD or DTS-HD Master Audio soundtrack, were you so inclined and were it even possible.
More importantly latency between my living room and router dropped from some 50ms on 802.11g Wi-Fi to a mere 1ms with the powerline Ethernet kit. Gamers among you will appreciate that this could prove the difference between virtual life and death in a heated deathmatch. Don't worry, you can still pretend to be using wireless if you die - nobody can tell otherwise.
As ever, this HomePlug kit proved incompatible with my old set, which is admittedly a couple of years old, but should work with any HomePlug AV adaptor such as a Solwise HomePlug AV Mains Pass Through or the aforementioned HDBX101. Considering four devices can be used with this kit, that's not likely to be necessary though.
With transfer speeds as close to the theoretical maximum as can actually be achieved in real circumstances and the convenience of a four-port Ethernet switch at its business end, the Netgear Home Theater Internet Connection Kit is one of the best HomePlug systems we've looked at.