There were major problems early in the DP cycle, but they resolved themselves as of DP4. However, you do need to have the standalone command-line tools installed to populate /usr/include: it's empty otherwise, and Xcode doesn't provide a CLT package anymore (edit: not entirely accurate, see comments below).
Once /usr/include is populated, you shouldn't have much of a problem with most Homebrew packages.
It does, but it doesn't install them under /usr anymore (unless you tell it to by calling `xcode-select --install`). See the homebrew discussion on the subject: https://github.com/mxcl/homebrew/issues/20427
You're right: Xcode includes its own set of CLTs namespaced under /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/, but that's useless for anything expecting FHS.
What xcode-select --install will do is the same thing as downloading and installing the standalone CLT package manually, but in a manner similar to how X11 used to be distributed: OS X will request permission to download the standalone CLT package and install it: http://i.imgur.com/O1CIGCP.png
It also doesn't currently work (it'll error out saying it's not available on the Software Update Server). Assuming that's fixed by release, it will save a trip to the web browser to download it manually.
Once /usr/include is populated, you shouldn't have much of a problem with most Homebrew packages.