It is a little hit 'n miss though I suspect they're better now, rather than when I bought one for my boyfriend from Maplin a few years ago. It didn't work at all (can't remember the make but it was less than 5.8GHz). After some more investigation, I got one from an online auction site, called
5.8GHz Wireless Audio Video Sender Receiver Kit Nikkai
which works perfectly. I think if you stick to (a minimum of) 5.8GHz and plenty of channels to choose from, you won't go far wrong.
As far as changing channels go, the "magic eyes" do need to be pretty close to the "spot" on the tvs where the remote is pointed (if that makes sense ?!). And there's no way you need the "sender" tv on !