I am in no position to estimate the possible ecological damage done by these tests (and I'm afraid neither is the French government) and so I oppose the tests on purely political grounds.
The Nuclear Non-Proliferation Treaty (NPT) has recently been extended indefinitely. The signing nations that not currently have nuclear weapons agreed to never acquire them; the nuclear powers agreed in exchange to reduce their arsenals and enter negotiations with the ultimate goal of complete elimination of these weapons. In addition, they agreed to provide the have-nots with the means to peaceful use of nuclear power.
The French action, shortly after signing the treaty, shows that France sees the NTP as nothing but the cementation of the power of nuclear countries. Nuclear tests in order to develop more powerful and/or precise nuclear weapons are obviously not done in the spirit of eliminating those very weapons. France wants to remain a nuclear power and it doesn't want other nations to acquire the same status. This is nauseating arrogance. It is also stupid, because it might well lead many countries to the conclusion that ratification of the NPT does nothing but belittling their own options.
To be fair: China and the United States show the same arrogance and stupidity. China has never stopped conducting nuclear tests and the US stopped them only after having acquired the ability to conduct nuclear bomb test simulations in a laboratory.
But France has counted itself to the civilized countries up to now (albeit one engaged in state-sponsored terrorism - never mind). This and the fact that they conduct their tests not in their own country but in a colony makes their action much worse.
Strategically speaking: international protest won't change the position of the French government -- they consider themselves to be a world power after all. The resistance has to come from the French people themselves. But how to induce this opposition? Discuss the matter with your French friends and don't buy French products.