Bear in mind there is no safe level of mercury, it's considered harmful in any amount, the degree only increases with exposure, and it is cumulative with a very long half-life in the body. Put a lamp into landfill and who knows where the mercury ends up.
I had high hopes for energy-saving lightbulbs when they came out, now I think they're an utter disappointment. Apart from poor light levels, slow warm-up times, expensive embedded energy in their construction and over-optimistic claims for their energy-saving ability, they are pointless or even useless for many applications.
And in winter, they may not save any energy in any case, in any house where the temperature is thermostat controlled, as they warm the house, thus saving on heating costs, hence one reason why the claims for their efficiency are exaggerated.
I did hear of one shop getting round the law by selling 'electric globular heaters' instead of incandescent bulbs.