Anyway in most cases you can start with buggy SensorTower to find out what keywords will bring you more relevant traffic at lower competition. After than just keep tracking things with a free AppAnnie ASO tool.
[1] http://www.androidb.com/2014/07/10-tools-to-help-with-google...
what I mostly need when doing ASO is:
1. Traffic estimation 2. A fast way of researching competition for a specific keyword
SensorTower gives us both and results from updates suggests that there are a connection between their estimations and true life.
How do you experience AppAnnie/MobileDevHQ traffic estimation tools?
We've got competitor tracking, support for other languages and data export (downloading csv with the search phrases+results). We also support the pre-launch analysis :) There's also an AppStore prediction tool that is quite unique compared to the competition ( http://techcrunch.com/2012/03/12/appcod-es-launches-app-stor... )
However, the approach isn't totally unscalable, as you can work your way up to targeting more and more trafficked keywords as your app grows.
An exptremely important part of ASO is to keep on doing it every time you update your app.
But a lot developers just ignore ASO, which opens up possibilities for those who take it seriously.