Surprisingly, I would like to add a few cons now, just to keep things balanced:

* Since Ruby and RoR are still relatively new to being used on a large scale, it would be best to avoid trying to build large business-critic applications on it just yet.You don't want to reach to a point where your servers will just crash and have no backup immediate plan.We are still waiting on gathering enough experiences from programmers on Ruby's scalability and efficiency on such critic mediums.

* Ruby is great tough for projects that have a well defined scope and that do not need to sustain tens of thousands of users.It appears that at least for the moment,Ruby is slower than php or asp...this could also result in you having to add more resources to your system than you would have to in the case of php or asp or java.

* Shared-hosting that is Ruby ready is still not widely available.You will probably need a dedicated server or a VPS if a shared-hosting account won't suit your space or bandwidth needs or if you want to modify the standard Ruby configuration deployed on the specific shared hosting server.