After using Carbide C++ for about 8 months, I think Carbide is not so bad. For my main open source project (Symbian bible), I still prefer Linux with my custom Makefile. The good thing about Carbide is that you can easily debug your project in the emulator. This is still impossible to do in Linux. For the commercial version (Professional onward) you can even debug on the device.
For the commercial version, the GUI designer is quite OK for many kinds of applications. You can create dialogs in minutes which usually takes hours. The GUI designer will not be too useful if you are building games, or applications that only have one view ("one screen"). The GUI designer is extremely useful for developing application with many data forms. As for the advanced features like performance tools or tools for the OEM, I have never tried it.
So is Carbide C++ for you? This tool is useful at least for one thing: debugging using emulator. If you need to do on device debugging or designing many applications with many data forms, the it might be useful for you to upgrade to the non free version. For me, I think I will do most of my project using the command line. If I can do debugging or use the emulator in Linux, then I would probably use Linux.
