Can you tell us about strikes and their effectiveness in France?

Before answering your question, I want to point out a few things. I have noticed that some foreigners, especially those coming from countries where those things are not common (i.e. Americans), tend to confuse three things.
They are the strikes, the protests and the riots.

I assume you all know what a strike is, but just in case: a strike is when workers decide to stop working because they’re unhappy with something in their work, coming either from their boss and/or the government.

A protest is when people “take it to the streets.” People that go on strike usually tend to protest too, in order to show they’re on strike for example, but there also can be a protest without a strike, for example when people protest for or against something that’s not related to their working or living conditions, for example a protest against war.

A riot is some sort of a violent revolt. It can be a protest gone wrong (student protests have a tendency to end into small riots), but it doesn’t have to be a protest, you can have riots for many reasons, the most common type these days are soccer hooligans street fighting outside of a stadium, and more important, people from poor suburbs rioting usually after incidents involving the police.
It’s a common misconception that protests always end in riots or have a “rioty” nature, but this is far from the truth, most protests are very peaceful or even festive.

Now, to your questions.
Do strikes are still effective?
Of course they are. If they were not, people wouldn’t go on strike. Do you think they do it for fun?
When the government wants to implement a new law related to work and if that law is unpopular, it always works as follows:
The unions say they don’t like that, and will threaten to go on strike.
The government will tell that they can do what they want, it won’t back off, this new law is good, it will be voted and implemented, whether people like it or not.
The unions and other people will go on strike, usually with protests.
The government will stay it won’t back off.
The strike goes on, more protests will be organized.
The government will call the unions to negotiate.
The government will back off.

Then, there’s this assumption that “they’re so frequent,” but that is a common misconception.
Actually the French don’t go on strike more than other countries (believe it or not, US included, sadly I can’t find official numbers, but they’re out there), but ours are so famous, maybe because they are an effective and “normal” way to put pressure on the government.
They are also more “visible”, by that I mean, that in many countries (US included) people from a company or a profession will go on strike to pressure their boss, not the government, so they’ll be more localized, and attract less media.
Also, in the US, strikers won’t go on massive protests, mostly because they’re fighting for their own rights, whereas French people tend to do it for the whole population.

Concerning Sarkozy, since he’s been in power, he has that will to break up the strike culture and the unions, a little bit just like Margaret Thatcher did in England. But if Sarkozy is as evil as Thatcher, he’s definitely not as competent, and so far his despicable attempts have backfired: he tried to pit the “normal people” against the strikers, almost implying they were some sort of terrorists (taking “normal” people hostages of the strikes, and such), which angered and mobilized more people against him than against the unions.
And then his infamous “nowadays when people go on strike, nobody notices” was partly responsible for the biggest protests in France since 1995 (and believe me, everybody noticed), protests not directed against any law in particular, but against Sarkozy himself.

How do I react when a strike breaks out? Well, these days, I usually hope it’s gonna be really big, that the government is gonna mess up badly dealing with it, and that it’s gonna crash in the process.
I don’t personally go on strike myself because I can’t (my employer being an American University it wouldn’t make much sense), and let me underline that unless it’s a strike that affects a particular profession (like truckers or farmers for example), people working in the private sector rarely go on strike, as it’ll be perceived as going directly against their boss, not against the government. So unless the goal is indeed to fight your boss, you rarely go on strike in the private sector. In the public sector… Well, your boss is the government, so…
Then, if there’s a big protest and I can go, I’ll make sure to be part of it.
I haven’t always reacted like that, before last Winter I hadn’t been in a protest in 9 years (and in more than that in France, 9 years ago, it was abroad and off-topic here), but the current government is pretty much the worse thing that has happened to France in recent memory (or even since the 4th Republic), so yeah, I’ll confront it as often as I can.

I’ll finish by telling you that if you’re in France in October, a anti-government protest is planned on October 7th, will it be accompanied with a strike, I don’t know, but I assume so (but just a daylong strike).