TLDR: Its a lot of fun, but not worth it for casual medical fetish folks.
Stethoscope background: I have used (professionally and fetish-ly) Sprague Rappaports, Littmann Lightweight II, Littmann Classic II and III, Littmann Cardiology IV, and now I have a Eko Core for my Cardiology IV.
I have auditory processing disorder and tinnitus, so at work the Eko Core has been a big help for me. I don't use the Eko app at work since I have no need to record anything. Recharging it isn't a big deal to me; I can even do that at work. Never had it die at work even going days without a recharge.
For fetish related things, its more subjective. I love that you can record and replay, and you can do wireless listening so that you play what you are hearing through the stethoscope through your phone. A quirk is that the free software doesn't let you use the stethoscope as your microphone for your video/camera on a smart phone or computer. A work around is you can use wireless listening and boost the volume of the eko and app while recording with video, though its not the best quality. Not sure if the paid app and software will let you use it as the microphone while recording from video. I guess the other option is using the paid software to record and layer it onto a video recording.
That said, the Eko Core is $250 US, so I think it really comes down if you are committed enough/ financially in a place where you can afford to spend that kind of money towards fetish-related "toys". If you are a die hard cardiophile fan or really committed to your medical-fetish play, awesome and go for it. If you are the major of folks where auscultation is simply a part of the med-fet experience, go for something more affordable in my opinion.