You guys should put up an example on how to convert a (small) test suit to use it, like you said you did in your top comment. The examples you have are cool but don't really help visualize that parallelism gain.
There's a section in the readme which I'm not sure of - "Running integration Tests for example is much faster." I guess that gains is from the parallel executions. But for the integration testings, there is still work to be done to make a central controller / runner to distribute various tests to different Lambda instances, correct? If that's the case, is loadtesting using Chromeless be another use case? Just initiate in parallel and blast the app infrastructure. It won't be efficient in terms of costs as using non-real browsers but this is probably as realistic as simulating real users load through real browsers.