First, I needed motivation to switch - “the why.” And the why is simple: speed. Mongrel and mod_proxy often have far better performance than a FastCGI setup (although this may depend from setup to setup, application to application). As better performance means less overhead, I was on board for learning what this was all about. Mongrel also has other advantages; most importantly, it is easier to debug problems (as discussed in the web-as-pipe essay). As I found out, it has only a marginal learning curve.
Do yourself a favor and use gem to install mongrel, cd to the root of your rails project and run 'mongrel_rails start' After another strong article and my own experience Mongrel makes getting Rails projects up really easy and it is faster for the dynamic portions (i.e. the hard stuff). If you combine Mongrel with a proxy layer from Apache, Lighttpd (wait for the next mod_proxy release), or Pound (specific proxy layer) for the static content you have a solid solution.