I am very nervous about sky marshals with guns on planes although I know El Al have had them for several years.
However, they have been dealing with a different type of threat - hijackers who hijack the plane and hold the passengers, crew and plane up for ransom, usually diverting the plane to another country... truly ghastly, but at least there was a hope of survival for most if not all on board.
I'm not so sure about dealing with a mind that is intent on destroying both plane and passengers and him or herself regardless - one shot on the plane could have the plane lose pressure and fall out of the sky.
I support the issues raised by the pilots - that captains should be in charge of the plane, should know that there is an air-marshal aboard and where they are sitting.
I also worry that to man any quantity of Transatlantic flights, they will need a large number of marshals - the more you have, the more risk you have that one makes an error, either in judgement or in aim and any mistake from them would probably be catastrophic.
These are my concerns re armed air-marshals - I realise though that steps need to be taken to try to ensure the safety of planes and those aboard, and if it stops another plane being used by a terrorist, then I'll be happy to change my opinion.