This is some years ago, maybe things have changed but I think the data coming back from the API is what confused our users most. Since it wasn't the full fire-hose, the API results were often inconsistent. Sometimes tweets you'd expect to be there wouldn't show, sometimes there seemed to be whole swaths of data missing from more recent days, sometimes it was date ranges, or sometimes it was baffling random. We dabbled with Gnip but unfortunately the pricing was way out of our range. Gnip seemed to be more aimed at large companies with deep pockets. Instead it just came a little used feature of our product that got more or less ignored. I don't think the Twitter API product was terrible, it just fell short of our users expectations and we couldn't explain our way out of it.