We use Java, MongoDB, Hadoop and embedded Jetty at MetaBroadcast, we try to keep it simple: no enterprise bloat etc. We've not had any performance issues.
We really like the support for higher-order functions, like Iterables.transform (aka map) and Functions.compose amongst others. There are loads of useful tools including MapMaker, which is great for building caches. The downside is that Function<X,Y> is an interface and implementations are often inner classes, which can be syntactically quite messy.
We've recently open-sourced our common library, which extends Guava with a few additional features we've found useful:
http://www.doc.ic.ac.uk/kenya/