Someone on HN previously mentioned that every project they've started with Sinatra, they've regretted down the road that they didn't use Rails.

I think the complaints about Sinatra are vaild, in that it needs to limit how params are accessed, better startup documentation, etc... But ultimately Sinatra is for building super simple crud apps. I don't even use it for anything that accesses something more than a SQLlite DB. I often use it for serving up basic front end assets and running single page applications that have some small server side component to them.

It's footprint is lighter than rails, but so is it's intended functionality.