Actually I am afraid to host a RoR app on my server, which runs several Java apps. I think the server has only 256MB of RAM (it is old, but cheap), or maybe 512 - not a lot, in any case.

I am much more worried about memory consumption of RoR. Never really launched it in production, but wasn't it the case that you have to load balance between several Mongrels, each consuming > 100MB of RAM?

Java is just unpleasant to use - other than that, it is very fast and memory consumption is ok.