for me it's memorizing. i have loads of music stored in my brain that i can play at will. i only took 3 months of piano lessons then quit, then 3 years later i started to teach myself so i cant sightread so i go through and learn it note by note by memorizing.

I can't memorize anymore. So I have to have the music. Some unfortunate accidents resulting in head trauma are what I attribute it to. I couldn't even play twinkle twnikle little star. I could figure it out and play it, but I'd have to do it all over again tomorrow.

If you want to memorize, you must be 100% sure you must be absolutely sure that you know the piece off by heart, otherwise that 1% chance just might kill you during your performance

I prefer to memorize, but have a copy of the music in front of me just in case I screw up.

Memorizing. Not only on piano, but on any other instrument. It makes it seem so much more like music to me instead of a bunch of little black notes on a piece of paper.

I tend to do it like Dan, but of late I've been trying (key word there) to play some more challenging pieces so I've been forced to read the music. I know I'll never be able to sight read and play Kieth Emerson's "Karn Evil 9: 2nd Impression", but I have fun.

I disagree that memorizing is more "musical" than having a score - memorized pieces will mutate with time, but a score is constant, and contains far more information than "play the riff like this"...but there is a point where relying on the printed music (especially in pop and jazz) can be stiffling.