As a San Francisco resident, and daily commuter to downtown, I find this app indispensable. If I only used it on my weekday commutes, it would be more than worth the $5 -- that's 1/4 what I spend on Muni fare every week!
The app has been updated regularly since I bought it (1.0, baby!), with the features and user interface getting better in every release.
When you think about it, the data used by Routesy and other Muni apps is all from the same source, NextBus. The data is free, and available. What you're paying for in an app is the user interface, the immediate usability. Routesy's lookup interface is great, and the bookmarks feature -- saving my favorite Muni lines and stops -- makes my daily lookups basically instant. Routesy is pretty close to perfect.