They work, but at the expense of top end efficiency. Any time a divided housing is paired with an open manifold you have turbulence where the two meet. This leads to less effective air flow and higher temps. Think of a set of double doors that lead out of a building, with a divider in the middle. If you have a crowd of people trying to run out through these doors and 20% of them are bumping into the divider, then bumping into others. The result is more time to get everyone out. Now, that analogy only applies to the higher RPM range.

I would personally rather have a good fully divided manifold and turbo nearly every time. There are only a few times I would have a divided manifold, and street driving isnt one of them.