The sensor in the new Pentax K-5 has taken over the #1 ranking for APS-C sized sensors on DxOmark.com, and not by a small margin. DxOmark is unambiguous in its praise: under the heading "The best APS-C in all tested fields," they write, "No need for suspense: this new 16.3 MP sensor is simply the best APS-C we have tested so far, sometimes able to compete even with very high-end full-frame cameras." It comes in sixth overall—nestled in between the Nikon D3s and D3 and surrounded by nothing but full-frame and medium format cameras. You have to go all the way down to #19 on the list to find the next APS-C sensor camera.
Calling its dynamic range performance "wonderful," DxOmark says this area is "clearly where the K5 struts its stuff...even the [Nikon] D3X’s full-frame sensor is not [as] good."
Where high ISO is concerned, being a "small" sensor, the K-5 is still "roughly a stop behind" the $780-more-expensive Nikon D700 and $900-more-expensive Canon 5D Mark II*. However, the K-5's overall sensor score of 82 trounces the APS-C competition, "more than 9 points better than the D90 or the Alpha 55, and 16 points ahead of the Canon 7D or 60D," and is top-ranked for high ISO among its same-size peers. (Note that the scores for the new Nikon D7000 are not posted yet, so Nikon still has a shot to retain the high-ISO crown.)