I've been trying functional ear training for a few months now. Functional ear training is about training your ear to recognise a note by it's "function" within a tonal piece of music.
For example, within a tonal composition in the key of C, the note "C" is the "home" note, the one you start from and return to. In tonal music, going back to the root note is the easiest way to make it clear you've come to the end of a specific melodic phrase.
The functional ear training app I've been using (maybe 5 to 10 minutes, every other day or so) works like this: it plays a cadence (a succession of chords: IV V IV I), and then plays a note. Given the chords you've heard, you have to be able to tell which note was played. This is made more straightforward when using "moveable Do" notation - using the "Do, Re, Mi, etc." note names, where "Do" is not fixed but always represents the root note of the current key.
I have progressed at it, there's no doubt. But it's very slow, and sometimes quite frustrating progress ! I've been doing it for some months now (can't remember exactly when I started), and now I can identify the correct note played from a random octave in the key of C about 80% of the time. (If I stick to a single octave, I get 100% now).
Is the skill actually useful ? Ha, I'm not even sure ! There's a part of me that thinks it can't hurt. I want to improve my singing and musicianship, and improving my ear is probably not a bad thing.
There are other ear training methods - such as interval ear training where you learn to recognise the interval between notes. Having tried it a bit, I found it harder and more frustrating than functional ear training - so I'm sticking to that for now !