I can answer this. Lots of speculation and many things.
This merger will only save about $1 billion in efficiencies/synergies according to management. Is it really worth it? Not sure if it is or not, but saving $1 billion on $110+ billion worth of revenue (0.9%) doesn't really mean much. These contracts are negotiated on a multi-year basis. I don't expect to see the $1 billion worth of savings anytime less than within the next 3-5 years. Investors of CVS/Caremark have gained NOTHING since their own merger, but that's a long story that should be discussed another time.
Will the regulators allow this to go through? If I remember correctly, the FTC prevented ESRX from buying Caremark in 2007 due to monopolistic concerns, which lead to CVS automatically winning Caremark. After the merger is completed, they would be #1 and have a 31% market share.
This business will now be about a 31% dominant player, a semi-dominant 15% player, another player around the 8-10% range or so, with dozens of local/regional players in the 4% range or less that amount to essentially nothing.
Full Disclosure: I own stock in ESRX and have for many years. It's one of my top 10 positions. Do a search of "ESRX" here on the forums and you will see all my posts associated with it dating back to 2007-2008. ^_^
I doubt employers, patients, and healthcare plans will see any realized "savings" from this merger.
I am "neutral" on this deal and will keep my stock in ESRX. I wouldn't go rushing to buy MHS despite the fact that it's trading at a steep discount from the merger price due to regulatory risk since I already own ESRX and can use that as leverage, but that strategy won't work for you since you don't own it?