When I'm studying for exams/writing a practice essay or something --> no music, because I need to focus. Also I don't need the music to focus, because the pressues on...sorta works both ways, if that makes sense
but for more mundane tasks- writing notes, maths exercises etc, or just regular study when exams aren't coming up soon and their isn't much pressure to really buckle down and study hard --> I find that music playing helps me from getting distracted.
I usually have my mp3 during study periods at school as well, coz music is less distracting than people yelling and talking and stuff....I swear my school's library is like our social centre...
Oh and I only listen to more 'mellow' music, like stuff I'm not going to start dancing around to
....for me, Coldplay seems the optimal music to get me to focus