We've not put it into production yet (it's coming in the new version) but are putting it through its paces in staging and it's handling things well so are confident it's ready for our usage.

There are people on the mailing list which are using it for some fairly hefty things.

Usual case of YMMV and as with anything it's worth taking a bit of time to try out the various options and see how they stack up for your usage. A good thing about ES is that it's so quick and easy to get running that trying it out and doing some benchmarking is pretty trivial.