I generally use cgdb (or gdb -tui) but for some complex tasks, I use the ddd gui frontend. It's not a pretty gui and the usability is not great but the visualization it provides is excellent. I've used it for complex data structures and debugging numerical algorithms. Getting it to display structured data as you step through the code can be super helpful.

That being said, I hope that lldb will catch on and there will be some decent front ends for it. The GDB-MI (machine interface) used in debugger frontends is quite limited and clumsy. The lldb debugger should be a lot easier to embed in a project as a library (as opposed to using pipes and gdb-mi).