Suppose it's April 2015. Someone asks you, "Is Pfizer (NYSE:PFE) a good stock to buy?" At the time, the big pharma company's sales and earnings were falling as a result of its "patent cliff" -- loss of patent exclusivity for multiple drugs. How would you respond?

It's easy for me to answer the question, because I wrote back then that I viewed Pfizer as a solid income play but not a great growth pick. As it turned out, that response was right on the money. Over the last three years, the S&P 500's performance has beaten Pfizer's gains two to one. But Pfizer kept on paying a nice dividend throughout the period. Of course, it was also an easy prediction to get right, considering the challenges that Pfizer faced at the time.

There are different dynamics for the big drugmaker in 2018 than there were in 2015, though. Is Pfizer a buy now?

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Positives

Let's start by looking at the positives Pfizer has going for it -- and there are quite a few. Pfizer's biggest positive of the past is still a big plus: The company continues to pay out a very attractive dividend. Right now, the dividend yield stands north of 3.8%. There are many investors smiling every quarter when they get their dividend payments.

Those smiles shouldn't fade anytime soon, either. Pfizer uses only a little more than half of its free cash flow to fund the dividend program. When asked in November what the company's capital allocation priorities were, Pfizer CFO Frank D'Amelio put the dividend at the top of the list.

Pfizer also claims several drugs with strong momentum. Ibrance posted sales growth of 46% last year, with revenue totaling $3.1 billion. Market research firm EvaluatePharma projects that Ibrance will rank among the five best-selling cancer drugs in the world by 2022, with sales of more than $7 billion.

Sales for Eliquis increased even faster than Ibrance in 2017 -- up 47% to $2.5 billion. And that's just Pfizer's share. The company splits revenue for the anticoagulant with partner Bristol-Myers Squibb. Xeljanz wasn't far behind. The arthritis drug saw sales jump 45% to $1.3 billion last year. Pfizer also has quite a few other drugs with solid sales growth that isn't quite as spectacular as that for Ibrance, Eliquis, and Xeljanz.

More winners could be on the way. Pfizer thinks that over the next five years it can win approval for up to 15 new drugs or new indications for existing drugs with the potential for annual sales of at least $1 billion. By comparison, the pharma company only had five launches of blockbuster products between 2011 and 2016.

It's worth noting that some of Pfizer's opportunities stem from acquisitions. The 2016 buyout of Medivation gave the company prostate cancer drug Xtandi. The acquisition of Anacor allowed Pfizer to pick up promising eczema drug Eucrisa. Pfizer is likely to make more acquisitions to fuel growth. Thanks in part to a big tax benefit last year, the company claims a cash stockpile of around $20 billion. And it could have additional money to fund acquisitions if efforts to sell its consumer healthcare business are successful.

Negatives

Some of the negatives of the past continue to plague Pfizer, however. The drugmaker is still dealing with the loss of exclusivity for a number of drugs. Two of the biggest challenges for Pfizer are declining sales for blockbuster drugs Enbrel, which lost patent protection in major European markets a couple of years ago, and Viagra.

The company also faces headwinds for its sterile injectables business gained with the 2015 acquisition of Hospira. Product shortages caused by capacity constraints and technical issues have held back sales growth for the business unit.

Pfizer's pipeline looks pretty good overall, but the company will arrive late to the party in one key area. Pfizer and Germany-based Merck KGaA's EMD Serono subsidiary are partnering on Bavencio. The PD-L1 inhibitor won approval last year for treating metastatic Merkel cell carcinoma and bladder cancer. Bavencio is being evaluated in late-stage studies targeting several other types of cancer, including non-small cell lung cancer.

The problem is that there are already several powerful PD-1 inhibitors on the market for treating many of those same indications. And Roche has first-mover advantage over Pfizer and EMD Serono with its PD-L1 inhibitor, Tecentriq.