Not enough...

I have, I think, 3 of those on my phone. LOVE THEM... the problem is I almost NEVER remember to use them.

In my experience they are extremely reliable... all 3 that I have. BUT.... only when you have them listen to the original song. Humming has rarely worked for me. It HAS worked by humming a very, very, very few times but not reliable at all when used that way... at least in my experience. We tried it once at a band practice where the guitar players played the melody and it failed nearly every time.... but that was some time ago so they may be VERY improved today.