I use an e-cig, and yes, sometimes do so occasionally on board or at stations, though not if there's anyone in the seats immediately in front of or behind me, or if the carriage is reasonably busy regardless.
Whether anyone has noticed or not I don't know, but I've not (yet) been asked to stop. I actually wasn't aware that it was specifically banned on the majority of TOCs, so I may have to reconsider.
While I don't condone anyone being exposed to anything that may harm them unwillingly, I doubt my device falls into this category - it doesn't produce much vapour at all, I exhale in a downward direction to minimise the dispersion.
I'd be very interested to see the results of a proper study on the toxicity of this vapour, on the face of it, it should be virtually zero - the ingredients of the liquid are a common food additive, flavourings, and nicotine. Nothing that should immediately cause alarm...assuming the listed ingredients are all that is contained, anyway.