Recs.
Updated
Specs
Pros
Pro Translates to Java code instead of bytecode
All Xtend code cross-compiles to Java before bytecode. While it is transparent for normal usage, in case you are confused with what is happening behind a scenes of certain syntatic sugar or annotation processor, you can just check generated Java code to learn what was generated. Very helpful when learning Xtend or debugging active annotations.
Pro Short and expressive syntax
Going further than Java 8, Xtend provides full type inference not only in lambdas, but also in local variables. Implicit lambda parameters, short property access, elvis operators for null-safe access and many more provide a lot better noise-to-signal ratio when writing and reading Xtend code.
Pro Active annotations, including predefined @Data, @Property etc
Possibility to code processors directly in Xtend. It's a bit similar to annotation processors in normal Java, Xtend provides a very easy to use API for modifying and generating code based on annotations and the resulting code can be previewed fully in an IDE. Xtend also provides a set of predefined annotations (including @Data for Scala-like case classes), but adding your own DSL-like postprocessors is really easy and maintainable.
Cons
Con No clear support guarantees
While it is an official Eclipse project and there is a lot of community plus a bit of commercial support around Xtext (Xtend parent project), Xtend itself is supported by just handful of people as part of their paid business support for unknown customers.
There is a risk of hitting some bug and not knowing if/when it will get fixed. Paid support options are also non-obvious (or possibly non available).
Con Slow compilation on huge classes
If you happen to end up with having too big classes, Xtend code autocompletion will get slow at the bottom of the file - up to the point of it being almost unusable for 1000+ line files. While it might be not an issue in long run (you shouldn't have such huge classes probably anyway and it might get fixed in future releases), it is sometimes an annoying issue while prototyping a solution. This is an IDE/edit time issue though, runtime performance is not affected.
Con Lackluster IntelliJ support
Xtend is majorly Eclipse and Gradle based. There is a beta plugin for IntelliJ/IDEA, but at the time of writing this con it is clearly a second class citizen, which gives considerably worse experience than the Eclipse plugin (and a LOT worse experience than core IntelliJ behaviour). Hopefully it will improve in future.