Writing a synopsis of a novel is every bit as evil as everyone says, and much more difficult than writing the novel itself.
When writing a novel, you have the freedom to let your imagination run as wild as your characters do, and it the story wants to take an unexpected twist, you can allow it the freedom to do just that. Even if you have worked your way through revision number seven, tightening and elaborating the path of the story, you still have that freedom. You are still the storyteller.
A synopsis says that the story is done. You want the synopsis to tell your story as concisely and as appealingly as possible. You want the agent or publisher to read the synopsis, and even though they will learn the end of the story, they will still want to read the novel.
A good synopsis should be about three to five pages long, give or take a little bit, but never as long as the first chapter.
I find that it helps to have two other people read over my synopsis -- one person who has read my manuscript, and one who hasn't. The one who has read it (in my case my husband), can determine if it stays true to the story. The one who has not read it previously will be able to look at it with fresh eyes and tell you if the story is appealing.
I like to think of writing a synopsis as writing practice. It is a good opportunity to hone your skills for writing tight prose that will pull the reader in.