In the airline I work for, our cabin crew catch lots of people using them and the offender is dealt with internally in the same way as those smoking real cigarettes, (because we do tell the customer through the Ts+Cs that we don't allow them so it's a breach of conditions of carriage). Depending on your other behaviours you could expect to end up losing your return leg and having to book a one way seat with another carrier to get you home at your own cost. The only real difference with e-cigs and real ones in airline world is that I'm yet to see a police prosecution under the Air Navigation Order for using an e-cig, so at least you'd avoid a potential £5000 fine.
I sympathise to some extent, my personal view is that they should be allowed, (because they're way better than the real, stinky article, which we get enough of as it is), but as with all things even connected with smoking in name only, there is hysteria among some of the public and it's that which largely drives this. Airlines have had passengers light a real cigarette in the cabin because they saw someone puffing on an e-cig.
There is anecdotal evidence that they may set off toilet smoke alarms, but it's debatable. Some people try to be ultra discreet and use them in the seat without anyone ever knowing (there's a breath holding technique that some use that produces no vapour on exhaling, and it can just look like you're sucking a pen while doing the crossword) but whether you try and do that is up to you. It's a pretty expensive puff when your fellow passenger or a cabin crew member spots you and your inbound leg ends up being cancelled.