If you mean just the ring adapter... pain in the rear to remove once attached...
Also watch that it doesn't turn on the mount, as this causes panic in trying to remove it without breaking anything...
The M42 with the bottom of a bottle glass is thicker, earier to remove, but does soften the pictures.
Links in the siggy to the M42 adapter taken pics.
However, for experimenting with different focal lengths that are usually out of price range, or some really really crisp older lenses, this is invaluable.
So be careful, but enjoy !