Hi Paul,
If you look at the quick review I posted of the 28-105 when it and the K-1 first came out
[lens review], I got similar results fully open that lacked microcontrast and sharpness. The overall result when not pixel-peeped looks great, especially for closer shots with pleasant bokeh backgrounds. Though even stopped down the fine detail is lacking on landscapes.
For me it is very much like the 18-135 on the K-5: a convenient lens that can produce pleasing results some of the time at 16mp. The 28-105 on the K-1 has the advantage of 36mp from which to down-sample from, and hence a consequently better keeper rate.
If the weather is not great, I use the 28-105, otherwise I much prefer the results I get from old F series primes; they out resolve the sensor most of the time and have that certain Pentax magic.
On the plus side with other standard zooms, the 28-105 has light weight, small size, WR, fast/quiet focus and price. I have not got the D FA 24-70 to compare against, but I have got an old Sigma 24-70 that is much bigger and heavier, but a bit better in the IQ department.
Chris