Yes, asserts are evil.
Often they get used in places where proper error handling should be used. Get used to writing proper production quality error handling from the start!
Usually they get in the way of writing unit tests (unless you write a custom assert that interacts with your test harness). This is often because they are used where proper error handling should be used.
Mostly they get compiled out of release builds which means that none of their "testing" is available when you're running the code that you actually release; given that in multi-threaded situations the worst problems often only show up in release code this can be bad.
Sometimes they're a crutch for otherwise broken designs; i.e. the design of the code allows a user to call it in a way that it shouldn't be called and the assert "prevents" this. Fix the design!
I wrote about this more on my blog back in 2005 here: http://www.lenholgate.com/blog/2005/09/assert-is-evil.html
