The interface it's simple enough to use, so you don't get lost too quickly when buying domains or subdomains that are not available anymore! I also liked how they offer some free credits every month - nice touch :) If there would be something special about this service in terms of usability (maybe better reporting / insights), then maybe we could have another winner (in my opinion) here.
However at present time their price policy makes sense only if you plan ahead & pay upfront anyway ;-) So no real complaints except perhaps from being unable really enjoy discounts as much :-( Have fun with your new park names!! The best part about parkio is that it works with any site you want to use, so if your website doesn't have an API or something like park, there isn't anything wrong with using parkio instead of working with another service provider. There are some minor flaws i noticed while trying out parkio but nothing drastic. I used parkio since 2017 when I first started my business and haven't had any problems yet! It's been great because we've always known where our domains were without having to pay anyone else.