I decided to give Helma a go because I like the idea of being able to re-use server side code within the browser and also because it should make it easier for others to extend the web application with plugins without having to learn a new scripting language.
I've been using Helma NG, rather than the stable Helma 1, so my comments are specific to that.
The dynamic nature of the language makes a huge difference. Both Helma and Grails (which you mentioned) feel like a breath of fresh air after working in Java with frameworks such as JSF. It's much faster to get things done, but I do miss the debugger and IDE integration.
Helma NG takes a new approach to managing scopes and I've found it easy to organize code into modules like in Python.
Helma NG is still work in progress, so parts of it keep changing, which I've found to be a bit of a problem. However, since most of it is in JavaScript and the framework is very lightweight, it is easy to wrap around the parts of a module you want to remain fixed.
I prefer Grails' GSP tag syntax to Helma macros, so might try to add support for that at some stage.
I'm using CouchDB for persistence and have been writing my own abstraction layer on top of that, so can't really comment on the SQL/Hibernate module.
Performance seems good, but I haven't run any benchmarks.
The community is relatively small, but very responsive. The documentation is there, but there are few examples. However, most of the code is self-explanatory.
So, to summarize: Helma NG is perfect for what I'm doing, but isn't yet stable enough to be used in place of Grails for a more traditional webapp. Helma 1 might be more appropriate for that.
Like your experience with Helma, Grails has also been a breath of fresh air. My biggest gripe with building web apps in Java is dealing with all the dependencies, setting up a project structure and making all the frameworks play nicely together...with grails none of those headaches exist...all the stuff you'd usually use is integrated nicely into a project and with almost zero configuration. The community is very responsive and new plugins are released on a weekly basis. I definitely recommend any Java developers out there take a look.
