I was generously sent a promo code a few years ago, but only started a new C25K session this year. The app is fantastic and easy to use (I'm currently fast approaching the dreaded week 5...). It's so nice not having to run with a phone. Over the years, this has changed from a 3.5" 3GS to the current 6.1" 15.
I've encountered one bug which is reproducible for me. When listening to a podcast through Apple Podcasts, most of the coach voiceovers work fine (the audio is paused, the voiceover plays and the audio restarts from ~1-2 seconds prior) but the voiceover after the final run (immidiately before the cooldown walk) does not play. I get a brief pause in the podcast where I guess the voiceover should be but the podcast picks up a fraction of a second later (with no 1-2 seond rewind). Interestingly, the end of cooldown voiceover 5 minutes later does work.
Not a major problem as I know that this pause in the audio means I can stop running.
If I can get to the end of C25K and put in a month or so of 5ks I'll look to buy the new 10k expansion pack.