We heavily use elasticsearch as part of Kiln's infrastructure. We've actually found it to be an insanely useful tool: it scales extremely well, it's highly redundant, it's trivial to extend, and it is fast. We've managed to move every last bit of our search infrastructure (including source code), using over 100 million documents representing 182 GB of data, and we still get search times around 10 ms or less. It's so fast we've got an entirely new feature built around it coming out in the next few weeks. It's just that awesome.
The only downside we've hit with ES is poor documentation. In all other respects, if you need search, I can unconditionally recommend it.