I’m rather suspicious of the reviews for 3DGO3DPrintingModels. It’s one of the lowest-effort, least accessible UI’s I’ve encountered. Given the apparent effort and/or lack of sophistication, I would imagine it’s either the project of a very young/inexperienced developer or simply a cash grab. It’s not really any more useful than searching in your web browser—it’s actually less useful, as many basic features from the websites hosting these models are not accessible. But don’t take my word for it, the free trial was simple to start, and just as simple to end.
Dev: Your 3D viewer is pretty good but, especially for $30/year, I would expect more from such an app. The UI is confusing. If a button is inactive, it should change states to provide some visual indication of that. It was difficult/impossible to get any details for the models. Just all around, it seems like you have stuck otherwise useful websites into a worse wrapper. I’m not trying to be mean or anything. If this is indeed a good faith effort to provide a useful service, then I hope you’re open to constructive criticism.