In my preview of the earnings report from drive-in joint Sonic (NASDAQ:SONC), I said that we'd get the "first taste of what easy payments and decent weather can do for the top and bottom lines." It turned out to be a pretty tasty serving -- with a side of uncertainty.

Sonic hit the top of its internal 2%-4% same-store growth target for the quarter. Add in the effect of new locations, and revenues grew 13%, exceeding management guidance. Earnings came in at $0.31 per share, again at the high end of internal expectations.

That said, there might be a few grains of sand in our burger this time. Net income dropped year over year, though the company's repurchase of 25% of its shares outstanding compensated for that problem on a per-share basis. The company has spent considerable cash on credit card readers at each drive-in stall, but we haven't yet seen whether that upgrade will boost customers' average sales per order.

The food is getting more expensive to make, too. Prices on dairy products and soybean oil have a very direct effect on a company known for its milkshakes and tater tots, and those prices are expected to rise about 10% over the next few months. Minimum wages are about to increase, too. Costco (NASDAQ:COST) or Starbucks (NASDAQ:SBUX) won't feel much pain, since they tend to pay their employees way more than the minimum. But Sonic and McDonald's (NYSE:MCD) don't swing that way, and they'll feel the pinch.

Higher expenses become higher prices at the drive-in, a true litmus test of customer loyalty. Would you pay 10% more for your milkshake?

Sonic is a company in flux right now. An upgraded and fast-growing store system, some harsh market conditions, and the start of the crucial summer season -- how does it all add up? I don't know, and I'd hold off any serious investing moves here until Sonic's next report. There are too many questions here, and not enough answers.

Further french-fried Foolishness:

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Fool contributor Anders Bylund holds no position in any of the companies discussed here, but he does like burgers and shakes. You can check out Anders' holdings if you like, and Foolish disclosure can quench your thirst for reliable reporting anytime.