They are not really efficient. All they do is add an extra layer of security so you can encapsulate that specific process in a chroot with its own networking stack. Other than that the CPU is still fairshared and the memory still has the same limits as if you ran it without the container. It adds nothing but security.
There are container technologies (openvz, LXC) that offer lot more than "fairshared" to manage resources between containers. Plus the efficiencies of lower "virtualization overhead", because you don't have full OS in containers compared to VMs (consider full VM kernel, VM HW drivers, etc)
