I liked being able to think of an idea, sketch out a schema and have it up in (admittedly ugly) webpages in a matter of minutes...and on almost any machine in just the time it took to download the package (no crazy dependencies, etc.).
The fact that you could then evolve this into something production-ready (arguably), well that was just icing on the cake!
Still working in Sinatra does give you that fun feeling of old Rails.