As a test, I submitted two photos.
The first was a 1.2 Mb jpeg I took on my phone and converted to B&W. It was a squirrel on a tree in front of a building with grass on the ground. Although the result wasn't identical to the original (it had no basis for choosing exact colors), it looked realistic, sharp, and in general was very believable. The grass was green, the leaves were a different but nice shade of green, and the building had subtle and appropriate colors. Viewed side by side, while it was possible to tell which was the original, both looked good enough to post online or look at in a normal size on my phone.
The second was a rough test. It was a small, 61K Jpeg of an old, yellowed newspaper photo. This app successfully colorized all the people and their clothing in a 430K PNG. It rendered the background as mostly black and white. Even getting rid of the yellow was done well, and there was no loss of sharpness. This is not perfect, but it is impressive
All in all, this is a very good app. The upload and download times are short, and the colorizing is just a few seconds. I hope the developer will continue to work on this, because he has a good app that could be a great app. Also, both the app and the website are free.
I'd just like to add that the Dev is in Hong Kong. I hope that one day these worries are no longer a concern, but the Dev can no longer agree that the government in PRC China will not require them to violate their privacy agreement. Because all processing is on their servers, I'd be careful about uploading only photos that are not controversial or could reveal sensitive information (I suspect this would be very few photos). This is nothing agains the dev, but reflects the current political reality in Hong Kong.