In my hands the K-5 is a little
too good - it allows me to think it can do anything, so I shoot in near-darkness or other unlikely times & places. It has limits, all cameras do, but they are far beyond what I am accustomed to. With its sealed body and a lens I can trust, weather is no longer an issue. Cold isn't either since it is rated to -10°C.. but in western Oregon that isn't an issue unless/until I get my snowshoes out!
I believe the D300 has the same sensor as Sony's A700; I really wanted one of those but the bulk was distressing. When Sony reinvented all its ergonomics, they lost my trust so I fled to Pentax, and the K-7 / K-5 body was just what I sought. The controls look similar to the D300 but in new places, so that learning curve is yours to scramble up
. You still have your 14-bit raw images, and in two formats. Something similar to active D-lighting is inside if you want that, and battery life is very good, sounds similar enough. The K-5 has a movie mode decent enough for most of us, not a pro-level system but OK by me. I wanted a still camera with clips as an option, not a "movie SLR".
My favorite thing about the K-5 is its relatively compact size. I often say that it has more features per cubic inch than any advanced camera on the market. DxO likes it, owners and reviewers like it.. Pentax did great work on this camera. If you have a Pentax group near you it would be best to hold one to be sure it's what you can live with - you're more likely to find an owner than a store that carries these, sad to say! If not, most return policies are good ones so you'll never be stuck. It's hard to imagine not liking a K-5, but some people get set in their ways and refuse to accept a new way to accomplish the same thing.