Is it strictly an X-axis leveler, or does it give you Z-axis tilt as well? If so then it's a true leveler. Otherwise don't chuck the spirit level just yet. :-)
Like rbid said it's only an x-axis tilt leveler but I've never been in a situation where I wanted to level my camera on the z-axis as well. Tilting my camera about the z-axis would change the whole perspective of my image so I use it strictly as a composition tool.