Pfizer (NYSE: PFE) is at it again. After a couple of years without any significant merger and acquisition activity, the big drugmaker announced on Monday that it's buying Array BioPharma (NASDAQ: ARRY). But how great is this news for Pfizer shareholders? That's a tougher question to answer. Pfizer's share price slipped a little in the initial hours of trading after the transaction was announced. The problem for investors is that the Array acquisition won't pay off anytime soon.