I think it is fun to read along with the mystery quilt projects as they roll out, and particularly to check out the pictures posted by those sewing along. But, actually doing one without knowing the ending - nope, definitely not for me. For me, one of the things I really enjoy in almost most every project is planning out what result I aiming for. I enjoy tweaking things so that I can end up with something that is exactly what I want. So, giving up that control does not appeal to me.
Last fall, at my guild's quilt show, there were two quilts that someone had made and the blurb explained that they were from a Bonnie Hunter mystery quilt project - when the time came to put the two types of blocks together, the quilter felt they just didn't work well together. By making extra blocks, she was able to get two decent quilts out of it, but I imagine it was disappointing to try to follow the color guidelines and then end up something that didn't seem pleasing.
I could see myself sometime trying a *small* mystery quilt - say wall hanging size. There the time and fabric commitment is so much less, that a disappointing result would not be so significant.
One fun thing with reading along with the mystery quilts as they happen is guessing which fabric combos are going to more or less well together. It's particularly interesting watching someone get good results with a color palette that is one that I'm not very drawn to - I feel it's a great example of watching color theory play out, and I learn things. But for me, I'm more inclined to stretch my color skills by using a kit or example of a color combo that I would not create on my own. My favorite of this type is a Jinny Beyer quilt that has many colors and includes orange, dark pink and red pieces all touching each other. In my childhood, I was taught that those colors do not belong together (for clothes and such), so putting them together was radical. The result is a dramatic quilt that I love. But, I was comfortable doing the radical (for me!) combination *because* I knew what the end result would be.
I think another big factor is whether you are mainly quilting for yourself (and/or family) or mainly quilting to donate the end quilt. With the latter approach, it is not a big deal to end up with a less than stellar end quilt - undoubtedly there is someone out there will who love almost any quilt one can create.