Custom white balance is a wonderful tool - I had used it more with the K100 than the K10 - the cameras have different tones with AWB and I personally prefer the K10 over the K100 (for the most part, but not always - the K100 tends to be more yellowish than I like, while the K10 is a bit more blue). So yesterday was like re-discovering an old tool in the toolshed - and I discovered it's useful in far more situations than I had thought about.
The K10's white balance control has more options than the K100, including the ability to over-ride the camera's choice by making adjustments manually. The one time I tried it, I discovered that the LCD color was slightly off, and my calibrated monitor showed me that the camera was right while my "by-eye" adjustment was off. So for me the extra control with the K10 doesn't get used - I use it the same way as I do with the K100.
I find it easier to get things right in-camera, especially when it comes to color, than to try to correct things later. I do fine if there's something white in the picture (with ACR there's a color picker that you can use to select what should be white or grey, and the program will correct everything else. It's quick and easy, but assumes that there's something neutral to use. In my second picture, it's all yellow and green - nothing that isn't colored. I just have a terrible time trying to get what I want using sliders - I keep going too far one way or the other, or can't figure out what to add or subtract to get it to look right.