Jam Sessions is one of those music games you probably never thought twice about, but if you were planning on getting a portable music game, you might be right to walk away. There is some fun to be had, but this is not your typical music game.
This is not Rock Band. This is not Guitar Hero. This is Jam Sessions. It is not necessarily a music game, either. It is simply marketed that way. If you expect a game like Guitar Hero, walk away.
I prefer to think of Jam Sessions as a musical instrument. It comes with a manual or tutorial to get you started, and the 'game' portion of it is actually more like a teacher to help you hone your skills. Then, you actually graduate to the "Free Play" mode and create your own masterpiece.
You use a stylus like you would a guitar pick. You puck a string up or down to make a sound. To change the note, you hold down a button. When you let go of all buttons, the note stops.
Jam Sessions is a musical instrument marketed as a game. If you look at it as more than a game, you might get something out of it, but if you simply want a music game, you should look elsewhere. I give Jam Sessions a neutral 3/5, because it's a fun toy but not quite worthy of being called a game.