There are three rides on the top of the Stratosphere, Big Shot, Ex-scream, and Insanity. You can buy a variety of ticket packages for these 3 rides. I opted for the $36 unlimited package.
Big Shot: This is a free-fall style ride that involves sitting in a seat that is raised very quickly to the top of the tower and then drops back suddently to the loading platform, giving you about 1 second of free fall. It repeats this procedure with a slightly less drastic fall a second time and then the ride is over. At least the line moves quick! I rode the ride 3 times in about 10 minutes.
Exscream: The ride was malfunctioning the day I was there, but even if it was working correctly, I have to imagine this line would take forever. The ride essentially involves sitting in a chair that is jerked to to the edge of a platform making you think you are about to fall off the edge of the tower, repeated a few times. The ride only makes sense from the front two seats.
Insanity: This is a standard spinner ride, that just happens to be on the top of a very high tower. It is kind of boring actually, but the views are incredible.
In general I think Big Shot is the only ride worth doing unless you are insanely scared of heights. All the rides suffer from poor line management though. It's an OK way to spend an afternoon.