LLDB is part of the LLVM suite of tools that Apple is moving to, including Clang. There are tons of improved features, including improved performance. There’s a quick intro for GDB users here: http://lldb.llvm.org/tutorial.html
However…
You might want to take a trip over to the forums at developer.apple.com. There’s a fair bit of rumbling about issues with LLDB at the moment (in XCode 4.3.1). One nasty issue, which people from Apple have confirmed, is that viewing ivars while stepping may show you the wrong values.