This is exactly what I needed for my 5D mk II. This camera is great, but was created before Canon put in the built-in digital level on the 7D, 60D, and 5D mk III. So sometimes (like when operating a glide cam with the 5D), I need to see if it's level as I'm recording. So that's great. The bubbles are big and very visible so you can see if it's level from a few feet away.
The one downfall is that this level was designed to be slipped into the shoe on the top of the camera for a quick check of the level, then I suppose after you're done, slipped out and put back in your pocket. There is no way to lock the bubble down onto your shoe. So in the field, it's not something you can keep in your shoe at all times. There's too much of a possibility of it being knocked out of place. Fortunately, though, it fits in there rather snug. So for use with a glidecam, it's perfect. It won't fall out if you simply point the camera to the sky, but that and a slight tap and it will.
So, all in all, it's perfect for what I needed save a locking mechanism that most shoe mount accessories have.