abimelech:
I don't believe there's any problem with using both.
This is quite true. There's is no bad problems interleaving them. That said, I tend to use yaourt now, except for very new things, as our PKGBUILDs for Arch know about which C libs to install. See this video, http://www.youtube.com/watch?v=xt5RE147s0Y
I think of yaourt mostly just as a wrapper for cabal, that provides a consistent interface for all the dependencies on my system (just wish it wrap all the latest ruby gems that way, too). Oh, if for some reason yaourt happens to not have the most recent version of something you want, you can just edit the version number and tarball address in the PKGBUILD when it asks you.
Exactly. I think this is a perfectly acceptable mode of use. And meanwhile it means we get very good testing on the AUR PKGBUILDs , until more and more things move into pacman.