DxOMark, which does scientific tests of digital cameras and ranks them on the results of those tests, just finished testing the Pentax K-30. No surprise, it ranks exactly the same as the Pentax K-01, with which it shares the same sensor and software.
DxOMark ranks it 24th best, but only because they place tied results in alphabetical order--the K-30 is in reality tied for 20th. It is the 8th highest ranked camera with an APS-C sized sensor (tied for 7th, actually).
The K-30 shares the same Sony sensor with some other highly rated cameras, including the Pentax K-5 (ranked 10th overall and first for a camera with an APS-C sized sensor), the Nikon D7000 (ranked 17th overall and fourth for a camera with an APS-C sized sensor), the Sony A580 (ranked 18th overall and fifth for a camera with an APS-C sized sensor), the Nikon D5100 (ranked 19th overall and 6th for a camera with an APS-C sized sensor), and the Pentax K-01.
Why the difference? It's simply the way each camera is programmed to process the data. And the differences are extremely minor from one camera to the next. Even the gap between the K-5 (best APS-C digital camera) and the K-30 (tied for 7th best APS-C digital camera) is so minor that you cannot see the difference without a close side-by-side study of 100% crops.
In other words, real world, there is no difference with regard to image quality between the cameras mentioned here. They all share the same fantastic Sony sensor that outperforms most other APS-C sized sensors and even some full-frame sensors.
What the K-30 has going for it that may be a reason to choose it over the other cameras mentioned are ruggedness and design. By design I don't mean ergonomics or style, which aren't really all that important. What I mean by design is ease of use and the camera's ability to "get out of the way" of what the photographer is attempting to create. Pentax was very thoughtful with the K-30.
Even so, the $850 price tag for just the K-30 body seems slightly high. I think if you can find it discounted, it's a quality camera with good value.