I Once destroyed an old credit card, it was a satisfactory mandolin pick (In a pinch). I've heard mando players swear that the only way to go is, with a thick pick. It totally depends on the style you play, different picks lend themselves to different styles (I haven't read all the posts, I'm sure I'm just reiterating other posts).
Bluegrass players tend to use thick picks, I play Irish and Finnish and a medium does the job, without sounding boxy when it comes to chords. A thick pick does bring a unique sound from the mando but, it really doesn't work with bouzouki(I haven't tried with guitar). I like to experiment with new picks, I like a lot of the different feels and sounds they can produce
It all hinges on your instrument, technique, and/or other preferences.