I love this very old article:

http://www.dotnetjunkies.com/WebLog/roydictus/archive/2004/06/02/15133.aspx

Let me quote you this paragraph:

Misconception: This is too much work!

Ouch--the old argument that quality assurance is too much work. I agree that it's no fun, and that it seems to bog down developer functionality (just like that other thingy that's too much work: unit testing), but the fact is that it doesn't. Smart developers (that is, those who write unit tests and asserts) spend much less time debugging their code than others simply because they know where to look when something goes awry. An informed developer is worth two, you could say.

Adding asserts doesn't add much work to the load of development. It's not like adding an assert is as complex as figuring out a new sorting algorithm, and it doesn't take much time to type, especially with IntelliSense.

Just read the article. There is little more to add.