Are you in the market for a new pair of designer sunglasses this summer?

It's the season for it, and you can spend hundreds of dollars on your next pair of shades. Some Prada and Bulgari pairs will run you nearly $500, and that's if you don't need prescription lenses. Even more moderate design labels like Ray-Ban or REVO can cost a couple of hundred bucks.

Designer shades are big business, even in this economy. I keep hearing about the new age of frugality, but I'm not seeing much of it at the mall. Sunglass Hut's same-store sales in the U.S. rose 10.8% in the first quarter, pretty much erasing the slump in early 2009.

But are these expensive brands worth it? How much better are they, really, than the $25 pairs you can get in your local pharmacy?

Before you spend big money on your next pair of designer shades, here are six things you should know.