I had five X1/9s. They were truly great cars and I consider the 4C a true spiritual successor to it (as was mentioned). The X1/9 was not just a Bertone design, but specifically Marcello Gandini of Miura/Countach/Stratos/308GT4 fame, and there is serious design commonality among the X1/9, GT4, and Stratos (as well as serious parts bin raiding)
One of my X1/9s was also the reason I ended up being given an F40 for a track day. I had been an in-car coach for the F40 owner at a Ferrari National meet. I'd been working with him to try to correct some habits and improve his corner exits to better suit its power delivery, and also working on balancing the car during the entry phase.
I took him out in the X1/9 which, making only modestly extra power, was much slower than the F40. Yet, he saw and felt the concepts that I was trying to convey in his car, and after being driven around consistently very much on the limits of little X, he enthusiastically handed over the F40 to me for the next session while riding and learning as passenger. In the next session we swapped, and his improvements were dramatic. Experiencing is Understanding. I then also had a little time in it myself as a reward.
The 4C, to me, feels very much like an F40, with its raw "turbo-forward" experience, unassisted steering with minimal self-centering and caster (The F40 actually has a bit more stock caster), and direct braking feel (4Cs brakes are way better than F40).
Good times.