Having been in film since 1975, Pentax IST-DL, then K200D, then K5, I think the K5 is a real step forward. I use the TAV mode most of the time, fixing the shutter and the F, and letting the camera set the ISO.
After many years I have come back to a polarising filter, partly because I shoot a lot from a light aircraft at high altitude (up to 20,000 feet) and haze is a perpetual problem. With a polarising filter you lose a lot of light but the result is still great because the camera can deliver a reasonable quality even at 1000 ISO e.g.
this or (on the ground this time)
this or
this.
The lens I use for just about everything is the 17-70 SDM DA. The F4 is not a problem either; in fact I shoot mostly at F8 for best sharpness.