Actually I had a really horrible experience with a toy that was a variation of a penis ring. I was masturbating with it, and the build up of blood within the veins became so intense that one of my veins ruptured and the blood pooled under my skin about half-way up my shaft.
...That was my first experience years ago with cock rings.
However, if you get one that is flexible enough (I'm partial to silicone!), do not use it until you are already erect, and remove it immediately after the session is over, you should be ok.
-Emt