How do they compare to those who have?

As good or better, generally. For entry-level hires (which many of my coworkers were - like many big companies, Google likes to hire straight out of college), Google usually hires for smarts, and once you've been there a year or two, you can pick up all the solid engineering practices and company-specific knowledge. They have nothing to unlearn, which is unfortunately often the case with people who've come from more spaghetti-like codebases.

One observation though - and this is a gross generalization, and obviously doesn't apply to everyone. The people who never studied CS before college and never worked on outside projects tend to be less innovative, less willing to think outside the box. So they're more dependable engineers and often write cleaner code, but they're less likely to come up with a crazy idea or go beyond their established job description.

There are exceptions, of course. Wonder Wheel was done by someone who didn't program until he was a couple years into college, and was initially interested in being a filmmaker (and he's only a year or so out of college, so that's only 3-4 years total experience).