So I am in the midst of the knitting the Eco Highland Duo Hat pattern from cascade yarns and I will say that my first experience with double knitting is not for the faint of heart. I am well into the seventh row of the pattern and the pattern is completely different on each side, so I need to continually change focus as to where I am in each pattern and whether I am knitting or purling. Fortunately, since the purl stitches are striped, after the first row you are simply purling the same color into each stitch.
The fair isle side is a bit more complicated because while you are purling the stripes, you are also knitting the fair isle between the purl stitches. But at the same time, it is quite fun and exciting to see two patterns working up all at once.
I have to confess that the first few rows I had to take out several times because I miss counted stitches somewhere in each of the patterns but now that I can see more of the patterns, it is coming along faster. I recommend the pattern if you want a challenging double knitting pattern but definitely plan on using a lot of patience and focus for this pattern!
Free pattern: http://cascadeyarns.com/patternsFree/W454_EcoHighlandDuoHat.pdf