It's a bit too early for real benchmarks, as my OWB is still full of debug output that slows things down considerably, but the Mandelbrot test I'm using seems to be somewhere in the region of 5 to 10 times as fast as the interpreter. But keep in mind that this is just the 'baseline JIT'. Webkit has another level of JIT called 'DFG' that promises further speedups through eliminating runtime type checking when possible, amongst other things.

This graph was recently posted with the caption "Figure 3. Relative speed-up (higher is better) on the Richards benchmark from each of the three tiers." to a Webkit blog post. While this graph only demonstrates a specific benchmark on x86, it still gives an idea about what kind of speedups are possible.