Interesting idea. Are you trying to tap into the same psychological drive or whatever that has people buy a message on the big screen at sporting events? Do you have a "Will you marry me?" option? :)

It would be nice to see more of the available designs and functionality without having to download the app. Like do I need to have my phone out to prompt the message on the screen to appear? Or can I do it real sneaky-like with my phone in my pocket and nudge my fiance to look at the screen to see the "Will you marry me?" message? In general I guess as a potential customer it seems weird that the promo pictures all have the person's phone out showing the same thing as the screen. Like it takes the magic away or something, I dunno.

I think a video that pulls at heartstrings would help. Also being able to see all current locations on the website would be nice. Regardless it seems pretty impressive that you have random screens spattered throughout NYC.

I'm more technical but a lower level "how it works" (bluetooth, etc.) would be nice, address privacy concerns, etc.

Just some random internet feedback for you, good luck!

EDIT: Read the app store description which answers some questions.