Where's the API? Dropbox has a tremendous ecosystem going for it. I'm not sure how prevalent it is on Android, but pretty much all iOS text and PDF editors have Dropbox integration and their new sync SDK is only going to push integration even higher.

I can't say I'm a very heavy Dropbox user, so I don't have many complaints (if any) about their offering, but if you want to get into the space you have to address those complaints. A shiny UI and better pricing can only get you so far and there's no way they're enough to beat out a competitor as entrenched as Dropbox.