Here's the way I see it. The great "exogenous shock" that was supposed to shake up the campaign occurred and ... nothing happened. After the killings in Montauban and Toulouse, the speculation was that Sarkozy might receive a boost from newly security-conscious voters, and indeed, the first-round contest did tighten slightly, but this effect was really more the continuation of a steady increase in Sarkozy's share over the previous several weeks. Unfortunately for Sarkozy, the second-round polling has remained stable at about 54-46 in favor of Hollande for several weeks now. Hollande has not been shaken from his tortoise-like crawl to the finish line. Mélenchon seems to be peaking at around 15%. Together, the 3 second-tier candidates claim about 40% of the vote.
So we will go into the first-round with the top-tier candidates at rough equality, each getting somewhere above 25% of the vote, with perhaps a point or two or three advantage for Sarkozy. Then will come the Great Debate between the two rounds. Sarkozy will have to go all-in in this debate, meaning that he will have to try to shake up Hollande in the hope that the Socialist will commit some fatal blunder.
I doubt that he will succeed in this, although Sarkozy is certainly a skilled debater. But the voters they will be competing for will be Bayrou's, since 90% of Mélenchon's will go to Hollande, but only 60% of Le Pen's to Sarkozy. One thing you can say about the centrist electorate: "sizzle" is not what it's looking for in a candidate. Hollande's lack of charisma is not so different from Bayrou's. Of course centrists also like accounts that add up, so you can expect Sarkozy to needle Hollande about the 60,000 teachers he has promised to hire, and you can expect Hollande to dance adroitly around the issue. You can expect some fireworks over the règle d'or and Hollande's pledge to "renegotiate" the treaty. You can expect allegations of "irresponsibility" and reneging on France's sovereign promises.
But in the end, centrists are fed up with Sarkozy, and there is little he can do to diminish the ras-le-bol sentiment. And so Hollande will win, although I think the final margin will be closer to 52-48 than the 54-46 in the current polls.