Ohh i did that once, not fun. The reason is because you usually see the last few moments of your past life. It is really depressing and you never know what will happen.
A was a little girl, no more than 4 years old with long blonde hair. I was running through the tall grass, well it was tall to me, and i ran over to a swing set laughing. I was having fun and then i big black dog, with brown around his eyes, came running and barking. He Bit me on the leg and pulled me down, he then just kept biting me and jumping on me. Then the dog got pushed aside and a redheaded woman, short hair, crying over me, then it went black.
It was very vivid and honestly i nearly cried at the end, if you can scry to see into your past live(s) i would suggest is, because, for me, it was just depressing.