In this scenario, have I already discovered your service prior to the app's availability? If so, I (personally) don't pay much attention to media blurbs on the site of the service/app itself. If I'm already using Exemplr, I don't care whether Exemplr.com has a pull quote from Gizmodo about the new Exemplr app on the App Store. As long as your site/mailing list/whatever communicates why I might want to use the app for your service, that's enough for me to try it. A full-fledged review is a nice back-up for paid apps, though.
If I don't know about your service already and/or the app is not just an interface to a service (contrast an app that interfaces with a photo upload service to an app that actually manipulates photos taken by my phone's camera), then it's kind of a different story.
I feel like I'm coming across as a little difficult or that I'm being confusing. Does that make sense?