I like Codewarrior. I've been using it for a very, very long time so maybe it's familiarity.
I like the preprocess function that allows you to see what'll be sent to the compiler. Really helps when the code you've inherited is liberally littered with #ifdefs.
I like the disassemble function. Caught a couple of nasty bugs and did some optimizations of time critical code because I could see what the compiler was generating.
I like the fact that it generates CFM applications, so I can carbonize my code and get an application that runs on 9 and X. Project Builder will only generate Mach-O applications, which means OS X only.
I like Powerplant. It's got its strengths and weaknesses, but if you know what they are, you can get some really nice OS 9/OS X applications where you only have to worry about what your application does.
Project Builder (and tools) have improved. At the last WWDC, the various managers were waxing poetic about the improvements that were in part responsible for the speed increases in OS X.2.
But... What I don't like - because I'm not used to them - are the make files flying by in an edit window. Has a strange taste to it - Like Visual Studio. Build options dialog are just command line parameters. Setting optimization levels shouldn't be (in my opinion) opt=4, let me pick that from a menu, or a checkbox in a dialog. It's a Mac applications for crying out loud!
The final item is cost. ProjectBuilder and friends come with every copy of X. Updates and newer versions available at Apple's site. Codewarrior is not. (Can't find Visa bill with last purchase on it).
But if you're new to programming and want to get your feet wet, ProjectBuilder is a fine way to go as long as you know that the resulting executable is OS X only.
I hope I've given you an idea why I like Codewarrior. If you have more specific questions, just email me.
John