It plays well with JRuby + Warbler, sits on Rack for pre- and post-processing goodness when needed (without having to intertwingle with stuff in the app proper), and basically gets out of your way. I don't feel like I have to learn assorted ill-documented mini-DSLs in order to just write a Ruby app that sits on the Web.
#ramaze on freenode rocks, too.
It's the most Ruby-ish of Ruby Web tools I've used.