What are the advantages of using Rails for a GIS/mapping application?
- Probably not.
- None stand out.
Your time would be much better spent learning Django/GeoDjango. Django is similar to Rails in that it's a web application framework. It uses Python rather than Ruby. The geospatial functionality is much more mature than GeoRuby.
Ruby/Rails is a great platform to develop on but the spatial functionality isn't on par with Django.
i am currently developping a rails app with some mapping capabilities, and i really love ruby and RoR, but sadly enough there is very little plugins mature enough for a complex WMS/WFS service.
But i just want to add that the recent rGeo library does quite a good job with projections (proj4 bindings and ability to use other APIs), integrates smoothly with the ActiveRecord ORM (i use it with postgre/postgis, works like a charm), and has some nice tools to read shapefiles, encode/decode GeoJSON...
Strictly from a learning perspective, learning something new is always worthwhile. However, Ruby/Ruby on Rails isn't extremely popular in the GIS world. Because of lack of popularity, I would suggest you pick up another language instead, such as Python, if your goal is to learn something new. I don't think you'll find any GIS-specific advantages to Ruby/Rails, but it does certainly have an appeal when it comes to building websites.
As for projects, there are Proj4 bindings for Ruby and GeoRuby