First, let's take a look at efficiency and quality of SunPower panels. On the quality side, no one has been able to match SunPower. Third party testing from a variety of companies shows that SunPower panels are more durable and has less degradation than competitors under a variety of testing conditions. This is important because over 30 years the difference between a 0.2% and a 2% annual degradation in power output is significant.
On the efficiency side, no one can come close to SunPower. The company's new X-Series panels are 21.5% efficient and a new manufacturing plant that's under construction promises even higher efficiency. For comparison, First Solar makes panels that are 13.3% efficient and only hopes to be 17% efficient by 2016. Chinese manufacturers are making modules that are 15%-16% efficient and even their best cells can't top SunPower's module efficiency (a cell is a component of a module and therefore has a higher efficiency).
What's important to understand is that having the most efficient and highest quality solar panels in the long term is important. Installers will get more power out of each installation, lowering costs at the end of a system's life cycle. It's also important strategically because the solar industry has proven time and again that lowering costs is easier than increasing efficiency or quality. If I'm going to bet on a module manufacturer, I want one that makes the best, most proven product today, not one that's trying to play catch-up.