Middle Split flexibility has always been a weaker point for me, even when I was younger and hadn't screwed up my mobility from years of "traditional" strength training work yet, I never had a great Middle Split (and it wasn't for lack of trying as I would regularly stretch to try to improve it).
I still have by no means mastered the Middle Split course, but I have definitely seen noticeable improvements in pretty much every stretch in the series.
In the Front Splits course I am fairly close to mastering the various Pike stretches. Due to my proportions I'm not convinced that I will ever achieve say a "toe to chin" level of compression like some people are able to, but chin/foreahead to Shin is probably a few months away at most I would say. Pike has always been an easier stretch for me than Middle Splits though, so I had less far to go.
Hip Extension has also noticeably improved, but I still have a decent ways to go before achieving a full true Front Split. As Hip Flexor flexibility had become significantly compromised from too much quad strengthening, not enough (or no) true hip hyperextension work (partially due to false/bad advice that I had received from the Fitness industry) and as a result I had developed chronic ATP issues, I'm very happy with the progress I've made in that department so far.
The foot stretches can still be quite intense, but again, I've made significant progress with them and the squatted Seiza has really helped my relieve pain/regain mobility in my left big toe area which had been injured while training (actually getting demoed on).