Recs.
Updated
SpecsUpdate
Pros
Pro Supports tie-ins for Rack-like middleware
Rack is an interface used in Ruby frameworks used to group and order modules, which most of the time are Ruby classes, and specify between them.
Slim uses a simple concept for it's middleware. By wrapping HTTP requests and responses it unifies the middleware into a single method call.
Cons
Con Too minimal
While it's true that Slim is a microframework, it's still too minimal. When used for throwaway projects or simple prototypes, it's perfect. But in the long run, it becomes less and less useful and you end up in implementing a fully custom framework in trying to tackle all the missing features.
So in the end, if the project would go further than a prototyping stage it would be easier to just implement another microframework that has those missing features out of the box.