I agree strongly this is the best way to interview a candidate. It's even relatively easy to set up for a new grad or junior candidate who isn't currently working. They should of course also be local or have no family or professional commitments that make a week-long non-work trip difficult.

Sadly, many of the best engineers I've worked with do have families, are working, don't live in the same town, and would absolutely balk at the idea of taking a week away from all that to "try out" a new job.

Of course, there's a wide spectrum between "work together for a week" and "write some C++ on a whiteboard for 40 minutes" that gets dropped from most screening processes. That's the zone where I'm most interested in finding creative evaluation tools.