In [this interview](http://www.guitarworld.com/edge-u2-interview-memory-man?page=0,7), The Edge reveals that he uses Herdim picks, the blue .88mm Herdim picks. "GW: Are you still using Herdim picks, the ones with the dimpled tops? THE EDGE: Absolutely. They’re these really cool nylon picks. I just like the way they sound. They make a blue and a red one, but I like the way the blue ones sound. I hold ’em upside down, too. I never analyzed why, they just sound better."more
I started using these because I read that the Edge plays with them, holding them backwards to strike the string with the dimpled part of the pick. He says it gives the notes that distinctive ching sound you'll hear on U2 records, versus a smooth pick, which produces more of a ping. He's right. Try it and you'll hear it immediately. It's kind of fascinating how different the articulation of picked notes is when using a Herdim this way.
With that said, beyond that, it's as good a plectrum as I can imagine. It's hard but still has some give/flexibility; the smooth side is nice for clean chord strumming; and when holding the dimpled side as it was meant to do, the texture serves as a grip to help prevent one from accidentally dropping the pick while playing.
I mean, it's a guitar pick; it's pretty hard to screw up. So, nice job, Herdim. I guess.
These picks are perhaps most associated with Dave "The Edge" Evans of U2. He flips them over and plays with the ridged end against the strings. This is the primary enabler to produce the "chink" guitar sound evident on many U2 songs e.g. "Where the Streets Have No Name".
These picks give the player a "clear feeling" of the strings, which helps keep the guitarist's strumming hand in control. The one drawback is that any perspiration noticeably reduces the player's grip on the ridged end of the pick and could cause some sloppiness during a recording session or live performance.
If you want to play U2's songs like it is played you must have a very good delay, a VOX AC30, a good stratocaster in many songs. After this you must have this pick. Its the same pick used by Dave Evans (The Edge).