For example, if I want to install ghc (a Haskell compiler), but it first wants to install Perl 5.8. As if I didn't already have a quite usable Perl 5.8.6 on the disk under Tiger, no, the DarwinPorts want their own versions of it. And then, depending on the path setting, I have either the Apple-Perl or the one from DarwinPorts active. Quite stupid - I think there should be pseudo-packages in the DarwinPorts that then refer to the pre-installed versions from Apple.
This causes problems especially when I also install packages manually. Because then sometimes the Perl accessible via the path is used - and with active DarwinPorts, that is the one there. But this is absolutely not the desired effect - after all, the Perl in this case only got in because the port for ghc has a build-dependency. But I don't want to use the DarwinPorts Perl at all ...
For the same reason, I find all the Python and Ruby modules in DarwinPorts unusable: they automatically pull in a new installation of Python and Ruby and do not use the pre-installed version. Rarely stupid ...
As a result, you can only use DarwinPorts on an OS X box for well-isolated tools - which is a bit of a shame, because the idea and the implementation itself are pretty great. Only too little consideration is given to the already installed stuff.
By the way, I installed ghc simply via the binary package from haskell.org. It says there that it is for 10.3, but it also works with 10.4 - at least what I do with it. And it saves me from having to build all that stuff.