We use Helma for our (unlaunched) YC startup, and we love it.
It's faster than rails, and IMO nicer to develop with. Having all the Java libraries without having to code in Java is sweet. The ORM is really well-designed and the built-in object cache is super fast. The template system has its quirks, but once you get used to it, is also really sweet.
It's a shame helma wasn't documented or evangelized as well as rails was. I think it's a better framework that just didn't catch on because it wasn't marketed well.
Well when you launch, PLEASE make heaps of noise about Helma - I think it would be great for both you and helma to do this (kind of like rails was for 37S in a way). It really does sound sweet.
