Depends on whose version.
Behe claims that it is impossible for "irreducibly complex" biological systems to have evolved. From this he concludes they must have been designed.
Not defending Behe, but it sounds like "impossible to have evolved" is a decent definition of "irreducibly complex". So far, that's a legitimate topic of discussion. The question becomes, "is there such a thing as 'irreducible complexity' in biology?" It may well be possible to answer that question one way or the other--for example, to identify a genetic structure with the property that no mutation is viable, or some such thing.
The part where Behe begs the question is when he demonstrates that something is mighty complex, and concludes that it's irreducibly complex. He's equivocating on the definition of "irreducibly" and taking advantage of the fact that one possible meaning of the word is "super-duper".
Correction:
Behe demonstrates that Darwinism cannot explain the formation of irreducibly complex biological system and posits design as a workable alternative theory.
That's called science at work.
You have three responses:
1. Demonstrate a path by which darwinism can generate an IC system.
2. Demonstrate that there are no IC biological systems.
3. Provide an alternate path that is neither darwinistic or design that can explain IC systems.
A few good scientists have taken a shot at number 2. No one has looked at number 3. Number 1 is pretty much universally agreed to be mathematically impossible, so most "scientists" make up a fourth response: Attack the theorist.