We did this in Skype for awhile.

The common concern at the time was "what if the candidate cheats, getting someone else to do his/her takehome for them?"

The way we responded was by conducting the in-person interview at least partially around the takehome assignment:

* What made you choose this approach?

* Can you talk about the architecture of your solution?

* What do you think about the following alternative solution?

* Where do you see potential points of weakness in your solution?

etc.

Then it becomes pretty clear if the candidate wrote the code or not.