The details of the Sanofi/Genzyme deal that were leaked are that shareholders will receive $74 a share in cash plus a contingent value right (CVR). The CVR will receive a payoff based on the performance of Genzyme's drug Campath. I believe a very conservative estimate of the value of that CVR is $2.25, making the deal worth $76.25. Sanofi already received US and EU antitrust approval of the deal so it should close within a couple of weeks of signing.
I believe the merger spread is about 2.5% on a deal that is likely to close in a couple of weeks. I have taken a long position Genzyme. The risk is that the details that were leaked are wrong or that Sanofi backs out because of something it finds. When Sanofi announced they need more time for due diligence, Sanofi did not deny the details of the deal so I take that as confirmation. Sanofi knows that Genzyme's facilities are a mess and seems very intent on closing a deal. I do not believe they are backing out.