I don't know if anyone has posted much about this from personal experience, but I just got my first test result. Here is a down-sampled file at ISO 12800. I just exported from Lightroom and down-sized in PS. No cropping, no processing, no noise reduction except the default Lightroom slider setting of 25 Chroma. In camera NR turned off:
And here is a 100% crop of the same file, before down-sizing:
From the full-res file, I sized to 24" on the long side, with the Resample box UN-checked. I then made an 8x10 crop out of that, again with no resampling, and sent it off to my lab.
Just got the print, and it is freakin' beautiful. I have to get within about 3 feet to detect the noise. Even then, it's not objectionable, and a 24" print will not generally be viewed from that close.
Obviously, this is just a headshot, and for large groups, etc. where important detail is smaller, things will be different. But this type of shot is maybe 75% of what I do/sell professionally. And I sell mostly 5x7 and wallets, occasionally 8x10s, and sometimes 20" prints, but this is rare. It's VERY rare I print over 20" for a client, so this test was something of a worst-case scenario for me. What this means to me is that for much of my important work, I can print from ISO 12800 without even trying....without going to any great effort to reduce noise or otherwise massage the files. More importantly, I can shoot at 3200 without even giving it a second thought, for this kind of work. This, my new Pentax friends, is beyond awesome.
I reeally need to put this camera head to head with my D3 in some "real world" high ISO tests. Hate to say it, but I have a feeling the K-5 will win.