With Unity 2D being abandoned and only providing Unity + Compiz in Ubuntu 12.10 as the default desktop, the out-of-the-box experience for low-powered ARM hardware on the stock Ubuntu desktop is a mess.
Last month the decision was made by Canonical to do away with Unity 2D now that LLVMpipe is coming into shape as a decent Gallium3D software rasterizer inside Mesa. LLVMpipe has been talked about since its very early days on Phoronix for running OpenGL on the CPU. With this being the default software fallback now in order to support Compiz, rather than going with the Qt-based Unity 2D, it's okay for x86 users.
In the x86 world most hardware has open-source 3D drivers that ship "out of the box" in Ubuntu so you always have 3D hardware acceleration and never have to worry about this Compiz/Unity-over-LLVMpipe. If you don't have a hardware driver or have to deal with this software fallback for a few minutes until getting around to installing the proprietary graphics driver, LLVMpipe is okay. LLVMpipe is powerful enough on a modern multi-core x86 CPU to handle the compositing window manager, especially if you're running 64-bit software and your CPU supports modern features like SSE4 and emerging AVX support.