It's generally better to think of work items as loop iterations than actual threads. On a GPU, they in fact to map to threads, but on the CPU a task pool just pulls work items. From the applications I've developed, OpenCL running on a CPU seems to have more than acceptable performance and I've seen results competetive with OpenMP.
The drawback with OpenCL is that you have a more restrictive memory and programming model. OpenMP allows you to do fairly arbitrary things in comparison and works in a variety different languages. For example, recursion isn't allowed in OpenCL and true functions may or may not exist on the target device, whereas with OpenMP, you have a much looser programming model that lets you do anything you can do sequentially with a few parallel constructs.
All in all, I think if you're coding an application from the ground up, OpenCL is a good way to go, but if you need to apply parallelism to existing applications, OpenMP can oftentimes yield a solution as simple as adding a #pragma parallel for.
