So, many months ago I said we were doing a Proof of Concept (PoC) to see what sort of productivity gains could be had using Rails vs. ASP.NET.
We implemented 3 user stories around logging in to a site and accessing a web service for user privilege information. The PoC showed that there could be as much as 2.5 times the productivity using Rails in that simple example.
Over the course of actual development, it looks as though 10-20% improvement is more likely as one gets to the actual complexities of a modern web site. It appears that Ruby encourages more refactoring and refitting as the code base grows and the team learns new patterns and stretches old ones.
As Rails matures, practices around it coalesce, and developers gain experience, I’m sure that real productivity will improve but at the moment, it ain’t no silver bullet.