It should be a bit more detailed than a pre-war telegram, I guess, but not quiet telling the entire story of the book. Then people dont have read the book anmore. Or they dont bother reading the synopsis if they see its too long to read it quickly.
So yeah, ideally let someone else write it. :)
Sadly, letting someone else write it isn't in the cards! Of course, a synopsis isn't meant for many people to read--just editors and agents--so having people not read the book because of one isn't a huge problem. I read just yesterday that many editors don't read the entire synopsis anyway--they pretty much judge based on cover letter and the first one to three chapters. Their world, their rules.
Hearing how they seem to operate, its surprising that they read anything at all. May be they just throw a dart at a printed list of author names. :)
The thing is, many of those grizzled old editors are so experience that they can tell in the first paragraph whether your work is good for them or not. So it had better be a darned good first paragraph!