Since I got it for free, it’s hard to say, but 20 in whatever currency is pretty steep for something that doesn’t really add that much to the free live timing. I did use it for every session though.
The most important good stuff is in my opinion: you see the tire selections instantly (don’t have to wait for the car to appear on screen or the commentators telling you), you can see where the cars are on the track (although the map is pretty buggy and the cars lag often), you can see why there was a yellow flag and who caused it, and you can follow a battle that’s not on the screen and “see” passes that are missed on the telly.
The bad stuff: doesn’t work that well on a phone with a ~4″ screen, a 7″ or larger (tablet) would be much more suitable for it. Can’t customize it enough (when you go from horizontal to vertical, you lose the option to see best sector times or top speeds).
I think Sky’s interactive channel(s) already cover most of this stuff, so if you have that, the app might be a bit redundant – then again, if you’re already paying for Sky you probably have the money for the app as well :P