I've always been a skinny-fat sometimes near underweight when I was younger (at the start of this was 5'8" between 130 and 140 pounds, haven't weighed myself in forever) and I haven't changed my diet at all since starting P90X. My results haven't been explosive or anything, I can do more pull-ups and pushups but my arms look about the same, but there's definitely been a reduction in the fat around my waist. My results would undoubtedly be a lot better if I followed the diet though. For example, I still have moobs even though I'm into the third month of the program and the amount of pull-ups I'm capable of has stalled at about three or four (and after about 20 minutes of doing arm exercises I get to the point where doing one is difficult). I have started to notice these two lines of muscle definition (I think anyway?) slowly form around my hips near the pelvis region though and while my pull-ups have stalled most of the other moves have improved pretty well since the beginning (flexibility is through the roof, I can touch my toes for the first time in my life). Even if the results aren't spectacular at least I'm doing something and my cardiovascular health is undoubtedly much better than it used to be so by that metric alone I think the program is worth it. Beyond that if I give it another few months (I plan on going through the program again when I'm done) I might be able to get rid of the moobs, who knows? Gotta exercise regardless and since I don't have a gym membership (and couldn't even stick with starting strength when I did at college) I might as well continue on with this.
This post reminded me, even if I don't look too different my body in a lot of areas feels a lot harder now, legs especially. So, yeah, still getting results just a little subtler than most.