A contract is a contract...it comes and goes. If you base your decision on whether it's going to be extended or not, you'll never really find any suitable job in contract sales...heck, with all the layoffs in BP, even that ain't safe.
As far as the Merck contract itself being good, yes, it is good. Most of the naysayers on here are actually poor, disillusioned folks who were expecting too much of a good thing. You will most likely get responses along the lines of:
* Merck counterparts are making life difficult for Merck contract reps
* PSS managers are micromanaging the sh*t out of their reps as if this was a full time regular salesforce in some BP company
* the pay sucks, no benefits, just a total waste of time
And possibly even more that I fail to mention right now...but don't let those negative things dissuade you from pursuing this - if it's really what you want.
This job isn't for those who expect a company car, full benefits, and a paycheck comparable to a BP rep. If you are currently a rep, then don't leave until they kick you out - then collect unemployment - then go after something like this. If not, then stay put where you are - unless your goal is to get out of Big Pharma. This job is for those looking for part time work, who are unemployed and looking for something just to keep experience relevant/current, or just for the extra income to supplement a main family income.