The way I've been explaining Padrino is that it's Merb built on top of Sinatra.
Merb was a lightweight, modular framework made up of a small merb-core and then merb-more, a collection of 20ish gems that provided various bits of functionality, most of which were problems that Rails solved in its core. Stuff like merb-helpers. If you wanted fancy helper methods, you required the merb-helpers gem. If you didn't care, you would not require that and have a slightly more efficient app. Similarly, it didn't have an ORM baked in--you'd use some gem like DataMapper (with an additional merb_datamapper gem that told Merb how to integrate with DataMapper).
Padrino seems to have the same setup of a minimal core with lots of modular plugins, but its equivalent to merb-core is sinatra. Which is awesome because sinatra is great and this way they don't have to reinvent the wheel.
Ive played with PAdrino a bit and its really quite nice. Very flexible and has a good communittee. One of the problems i have with Rails is that it seems to leave the other frameworks behind. In fact i only know of one or two places using something other then Rails. ANd its usually for backend stuff such as i think serving json. At the same time Rails is a heavyweight which not everybody needs.
