Thanks for your feedback and judgement. It's somehow coherent to what I thought evaluating it. While everything appears to be nice and well thought I kept asking myself: Will learning all that stuff (as well as Spring & other Java bloat) be really worth it? And if it is: Who develops in Grails anyway, will I find developers for my neat little Web 2.0 Apps?

So I will have a closer look at Sinatra / Spark which really seem a great approach to develop small-medium web apps. Previously I had my doubts about their capabilities (it appears too simple), but seeing the traction those technologies have might convince me otherwise.