Reasons to Use Grails

1. Clean rails-like architecture and design patterns but with an even cleaner API. In fairness, they got to see what did and didn't work well in Rails before they started.
2. Seemless intra-VM integration with the vast number of high quality Java libraries.
3. Easy to integrate with existing Java enterprise systems.
4. Runs on the proven, high performance JVM. The Ruby interpreter just doesn't compare in terms of performance. Yes, I know you can do this with Rails via JRuby, but that effort is still young.
5. Code by convention - escape the mess of XML config files in frameworks such as struts.
6. Use of Groovy, a Java-like language with all the scripty goodness you've always wanted.
7. GORM is great. Its finally as easy to define persistent data entities with types and constraints as it is for regular old in-memory data structures.

Of course there are drawbacks:

1. Rails has a much larger community.
2. There are many more Rails gems than Grails plugins.
3. There are many more Ruby books.