I've been looking for good data on iOS and Android apps. Can anyone confirm that Xyologic is a solid source of data and that the estimates are reasonably accurate? Is there a better source of download data? (appdata.com is good for usage data).
Haven't examined Xyologic. I tend to follow Flurry's announcements as they've had good adoption from reputable developers. Though this tactic only illuminates larger trends (app category usage).
For more fine tuned tracking, I track unique URL formats on Twitter to act as a proxy for usage and sharing. I'll capture tweets with specific URLs (like Instagram, Spotify, Photoset, etc) with Datasift for a day or week then calculate a rate of sharing.
It's a pretty tricky task, as no one has a clear picture besides the OS makers. Until an Alexa/Quantcast equivalent emerges I'm not sure it's worth the effort.