I haven't used Program-line ("P") mode on an SLR since 3 weeks after buying my *istDS, well over 2 years ago. I quickly found that Aperture priority (Av), and to a lesser extent, Shutter priotiry (Tv), allowed me to control the camera in a much more desirable way. Lately for more contemplative images I've been using the Zone system - shooting in Manual with spot metering - and using Av for quicker, kid-chasing type shots. The Pentax-specific TAv mode has been invaluable in variable, low light situations.
But today I was messing around with settings and remembered reading something about the K10D's P mode being referred to as "hyper-program" mode. Not remembering exactly what that meant, I played around for a bit. And then BAM! Suddenly I was loving shooting in P mode.
Hyper-program mode works just like any other program line mode - it best guesses at the Av, Tv (and optionally ISO) level, changing all variables to get the right exposure You can set biases so it will, for example, go for a short a depth of field as possible. All normal so far.
Until you hit one of the e-dials. Moving the rear e-dial allows you to set the aperture, which stays set until you hit the little green 'program line' button. The front e-dial controls the shutter speed in a similar way.
So essentially you can move from P to Av, P to Tv, Av to Tv and Tv to Av by just moving an e-dial. That's much quicker and easier than moving the mode-selection dial.
To be honest, it's not that often that I want to switch from Av to Tv. In fact I rarely use Tv at all unless I'm trying to shoot a moving object. But it does happen from time to time. What happens more often is handing the camera to my wife, which usually involves fiddling with a few dials to put it back into 'green' mode. Being able to just hit the green dot button to move back to full P mode will make that at least slightly quicker.
Despite the stigma, I think I'll be shooting in P rather than Av from now on. I just hope noone is looking at my EXIF.