Jochen has written an excellent piece titled Java 5 features in Groovy which describes what Java5 features can be found in Groovy (some of them as early as the first 1.1 beta), for example Enums can now be written with Groovy (albeit with some caveats) and covariant methods, definitely an interesting reading.

If you decide to try the outlined features and by any chance find a bug (gasp!) do not hesitate to file it at Groovy's JIRA, the Groovy team will greatly appreciate it.

Keep on Groovying!