I used these on Castle Rock in Leavenworth WA this weekend. I only have the 0.1 and 0.3 but they were solid and flexible. In two of my anchor builds, theses were the most solid pieces. Their expansion is noticeably better than the c-3's. I feel like the c-3's benefit from narrower head width, but that benefit is quite minor compared to the solid feel of the x-4's and the flexible stem.
All in all, sweet looking and more importantly a solid piece of gear. My trad partners are going to get them as well.