Don't you get a credit for that tax on your US income tax? Even with the tax the payout is attractive. The biggest concern should be the price the EU will choose to pay for their drugs. In an atmosphere of austerity even rich companies may have to face up to cuts.
I believe, but may be mistaken on this, that (i) you cannot claim any credit for the 25% if you hold SNY in a tax deferred account such as an Roth or regular IRA, since you owe no US tax on the dividends and, thus, have no US tax on the dividends against which you can apply the foreign tax credit, and (ii) if you hold SNY outside of a tax deferred account, you can clam a credit for the French tax only up to the amount of your US tax on the SNY dividends (i.e., 15% unless your AGI is above $400K, or $450 K for married copules, in which case it would be 20%). This means that you would have to eat the difference between the US and the French rates. You may be able to claim the difference as an itemized deduction on your Schedule A, but an itemized deduction is of less value than a credit and, anyway, you may lose some of the benefit of such a deduction due to the application of (i) the alternative minimum tax and/or (ii) the phaseout of deductions for higher income taxayers, if you are so "lucky." So it seems to me that the 25% French withholding rate is a big disincentive for US taxpayers to hold stock in a French-based company.