My particular K10 was an early model, and some people had trouble right off with the auto focus not being consistent (as I recall, that was one of Tom's biggest complaints). Mine seemed to be fine in the beginning, though it's always had a tendency to backfocus. From the charts I shot, I figured that mine would meet specs (barely) and there was no point in sending it back for calibration. And many of my shots are with manual lenses. However, its become really inconsistent now - sometimes it will say that it has focus when it really doesn't and everything is fuzzy. It's much worse with the DA*50-135, to the point I was ready to send the lens back as being defective, until I tried it on the K100 and discovered that it was right on with it, and a lovely lens.
I shot a bunch of pictures yesterday that were more comparison shots. I actually stood in one place and switched the lens back and forth between the two cameras. A couple of times the K10 couldn't find the focus when it should have, while the K20 had no problems. It only once indicated focus when it didn't have it. Where the focus wasn't an issue, the two pictures would look very similar until you looked at them 100% (then you see the extra resolution that you get with the K20).
Up until I got the K20, I had been thinking that the problem was me - I must be contributing camera shake to cause the occasional blurriness (even when the shutter speed was fast enough that I could hand-hold it). Or that the wrong focus was caused because the camera had focused on the wrong thing (which of course, does happen), and so on.
So I don't think the K10 is suddenly a lousy camera - I've taken hundreds of wonderful pictures with mine. I just think my particular one has an AFproblem that could probably be fixed if I were to send it in for repair. Since its no longer under warranty, I chose to replace it instead of paying to get it repaired.